efivar: fix build with gcc 7

Add upstream patch fixing a warning that breaks the build because of -Werror.

Fixes:
http://autobuild.buildroot.net/results/33a/33adc3ef139d6814aef4c92ae0bcc4c810ab0b86/
http://autobuild.buildroot.net/results/e7d/e7d80e823e13edc6698148244553bd90367bcd03/
http://autobuild.buildroot.net/results/3b6/3b61246f8b04a332d1c61732f0eb6e50ea8ca366/

Cc: Erico Nunes <nunes.erico@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
30 jobs for master in 82 minutes and 50 seconds (queued for 103 minutes and 40 seconds)
Status Name Job ID Coverage
  Test
passed check-DEVELOPERS #22073323

00:00:49

passed check-gitlab-ci.yml #22073322

00:00:45

passed tests.core.test_post_scripts.TestPostScripts #22073325

00:08:59

passed tests.core.test_rootfs_overlay.TestRootfsOverlay #22073327

00:01:18

passed tests.core.test_timezone.TestGlibcAllTimezone #22073328

00:04:37

passed tests.core.test_timezone.TestGlibcNonDefaultLimitedTimezone #22073329

00:05:22

passed tests.core.test_timezone.TestNoTimezone #22073330

00:02:32

passed tests.fs.test_ext.TestExt2 #22073332

00:12:24

passed tests.fs.test_ext.TestExt2r1 #22073334

00:05:36

passed tests.fs.test_ext.TestExt3 #22073336

00:07:49

failed tests.fs.test_ext.TestExt4 #22073337

00:01:17

passed tests.fs.test_iso9660.TestIso9660Grub2External #22073339

00:08:21

passed tests.fs.test_iso9660.TestIso9660Grub2Internal #22073340

00:16:05

failed tests.fs.test_iso9660.TestIso9660GrubExternal #22073342

00:03:13

failed tests.fs.test_iso9660.TestIso9660GrubInternal #22073344

00:04:14

passed tests.fs.test_iso9660.TestIso9660SyslinuxExternal #22073345

00:12:09

passed tests.fs.test_iso9660.TestIso9660SyslinuxInternal #22073347

00:11:44

passed tests.fs.test_jffs2.TestJffs2 #22073348

00:11:48

passed tests.fs.test_squashfs.TestSquashfs #22073350

00:05:24

passed tests.fs.test_ubi.TestUbi #22073351

00:11:17

passed tests.fs.test_yaffs2.TestYaffs2 #22073353

00:08:54

passed tests.package.test_dropbear.TestDropbear #22073355

00:03:24

passed tests.package.test_python.TestPythonBase #22073356

00:07:11

passed tests.toolchain.test_external.TestExternalToolchainBuildrootMusl #22073358

00:12:03

passed tests.toolchain.test_external.TestExternalToolchainBuildrootuClibc #22073359

00:06:49

passed tests.toolchain.test_external.TestExternalToolchainCtngMusl #22073360

00:03:11

passed tests.toolchain.test_external.TestExternalToolchainLinaroArm #22073361

00:09:48

passed tests.toolchain.test_external.TestExternalToolchainSourceryArmv4 #22073362

00:02:32

passed tests.toolchain.test_external.TestExternalToolchainSourceryArmv5 #22073363

00:02:22

passed tests.toolchain.test_external.TestExternalToolchainSourceryArmv7 #22073364

00:04:27

 
Name Stage Failure
failed
tests.fs.test_ext.TestExt4 Test There is an unknown failure, please try again

----------------------------------------------------------------------
Ran 1 test in 2.682s

FAILED (errors=1)
Uploading artifacts...
test-output/*.log: found 1 matching files

Uploading artifacts to coordinator... ok
id=22073337 responseStatus=201 Created token=-ffgw3nr
ERROR: Job failed: exit code 1
failed
tests.fs.test_iso9660.TestIso9660GrubInternal Test There is an unknown failure, please try again

----------------------------------------------------------------------
Ran 1 test in 96.785s

FAILED (errors=1)
Uploading artifacts...
test-output/*.log: found 1 matching files

Uploading artifacts to coordinator... ok
id=22073344 responseStatus=201 Created token=kTsszxZv
ERROR: Job failed: exit code 1
failed
tests.fs.test_iso9660.TestIso9660GrubExternal Test There is an unknown failure, please try again

----------------------------------------------------------------------
Ran 1 test in 67.509s

FAILED (errors=1)
Uploading artifacts...
test-output/*.log: found 1 matching files

Uploading artifacts to coordinator... ok
id=22073342 responseStatus=201 Created token=Cbshnrex
ERROR: Job failed: exit code 1