Fix missing function in Pg < 12 cases


git-svn-id: http://svn.osgeo.org/postgis/trunk@17302 b70326c6-7e19-0410-871a-916f4a2858ee
1 job for svn-trunk in 5 minutes and 21 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
failed #174337440
test

00:05:21

 
Name Stage Failure
failed
test Test
  ^~~~~~~~~~~~~~~~
geography_measurement.c:846:8: note: 'd2' was declared here
Datum d2;
^~
cc1: all warnings being treated as errors
make[1]: *** [<builtin>: geography_measurement.o] Error 1
make[1]: Leaving directory '/builds/postgis/postgis/postgis'
make: *** [GNUmakefile:20: all] Error 1
ERROR: Job failed: exit code 1