Add missing /snap empty directory in snap image

This is needed by snap to bind mount the image of the application. Without
it, running applications fails with:

```
cannot perform operation: mount --rbind /snap /tmp/snap.rootfs_AH4dGK/snap: No such file or directory
```
22 jobs for valentindavid/missing-snap-dir-19.08 in 151 minutes and 7 seconds (queued for 1 second)
Status Job ID Name Coverage
  Flatpak
passed #292642993
aarch64
app_aarch64

01:26:24

passed #291913580
armhf
app_arm

00:49:49

passed #292643018
x86_64 cache_i686
app_i686

01:28:23

passed #292643194
ppc64le allowed to fail
app_powerpc64le

00:55:10

passed #292643081
x86_64 cache_x86_64
app_x86_64

01:38:48

canceled #291913579
aarch64
app_aarch64

canceled #291913578
cache_i686 x86_64
app_i686

00:04:37

canceled #291913581
ppc64le allowed to fail
app_powerpc64le

00:55:11

canceled #291913577
cache_x86_64 x86_64
app_x86_64

00:49:30

 
  Vm
manual #292644485
aarch64 allowed to fail manual
minimal_systemd_vm_aarch64
passed #292644487
armhf
minimal_systemd_vm_arm

00:02:26

passed #292644488
cache_i686 x86_64
minimal_systemd_vm_i686

00:01:14

manual #292644486
ppc64le allowed to fail manual
minimal_systemd_vm_powerpc64le
passed #292644490
cache_x86_64 x86_64
minimal_systemd_vm_x86_64

00:02:11

canceled #291913584
aarch64 allowed to fail manual
minimal_systemd_vm_aarch64

canceled #292643112
armhf
minimal_systemd_vm_arm

00:00:51

canceled #291913585
armhf
minimal_systemd_vm_arm

canceled #292643124
x86_64 cache_i686
minimal_systemd_vm_i686

00:00:53

canceled #291913583
cache_i686 x86_64
minimal_systemd_vm_i686

canceled #291913586
ppc64le allowed to fail manual
minimal_systemd_vm_powerpc64le

canceled #291913582
cache_x86_64 x86_64
minimal_systemd_vm_x86_64

canceled #292643265
x86_64 cache_x86_64
minimal_systemd_vm_x86_64

00:00:37