gitlab-ci: install gettext

The gcc image has autotools, but not gettext, resulting in an
error about a missing .gmo file.
1 job for gitlab-ci in 51 seconds
Status Name Job ID Coverage
  Build
failed build #10007
docker

00:00:51

 
Name Stage Failure
failed
build Build
make[2]: Leaving directory '/builds/xiph/vorbis-tools'
tardir=vorbis-tools-1.4.0 && ${TAR-tar} chof - "$tardir" | eval GZIP= gzip --best -c >vorbis-tools-1.4.0.tar.gz
rm -f vorbis-tools-1.4.0.zip
zip -rq vorbis-tools-1.4.0.zip vorbis-tools-1.4.0
make[1]: Leaving directory '/builds/xiph/vorbis-tools'
/bin/bash: zip: command not found
make[1]: *** [Makefile:695: dist-zip] Error 127
make: *** [Makefile:699: dist] Error 2
ERROR: Job failed: exit code 1