gitlab-ci: Fail when C files are missing from POTFILES.in

If there are translation in C files but not in POTFILES.in
fail the pipeline.
7 jobs for master in 18 minutes and 49 seconds (queued for 31 seconds)
Status Job ID Name Coverage
  Build
passed #304882
librem5
build:native

00:05:50

 
  Test
passed #304884
librem5
check-po

00:00:30

passed #304883
librem5
test:native

00:03:20

 
  Package
passed #304885
librem5
package:deb-debian-buster

00:03:28

passed #304886
librem5:arm64
package:deb-debian-buster:arm64

00:06:53

 
  Test Package
passed #304887
librem5
autopkgtest-debian-buster-package

00:02:15

passed #304888
librem5
lintian-debian-buster-package

00:00:53