ges: Hide internal debug category behind a GOnce

Otherwise it might be used (e.g. by the plugin loader via the GES
plugin!) before ges_init() is called.
23 jobs for debug-categories in 60 minutes and 30 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Preparation
passed #686777
gst indent

00:00:20

passed #686776
manifest

00:00:40

 
  Build
passed #686787
build cerbero cross-android universal

00:14:51

manual #686791
gst-ios-12.2 allowed to fail manual
build cerbero cross-ios universal
passed #686788
build cerbero cross win32

00:11:05

passed #686789
build cerbero cross win64

00:09:09

passed #686786
build cerbero fedora x86_64

00:05:54

passed #686790
gst-macos-10.14
build cerbero macos x86_64

00:08:40

passed #686782
build clang fedora x86_64

00:09:17

passed #686778
build fedora x86_64

00:12:18

passed #686785
docker windows 1607
build msys2

00:11:00

passed #686779
build nodebug fedora x86_64

00:11:19

passed #686780
build static fedora x86_64

00:15:29

passed #686781
build static nodebug fedora x86_64

00:06:28

passed #686783
docker windows 1607
build vs2017 amd64

00:10:30

passed #686784
docker windows 1607
build vs2017 x86

00:11:10

 
  Test
passed #686835
check fedora

00:03:23

passed #686793
integration testsuites fedora

00:47:30

passed #686794
valgrind ges

00:07:21

failed #686792
check fedora

00:04:42

 
  Integrate
passed #686795
cross-android universal examples

00:06:52

manual #686797
gst-ios-12.2 allowed to fail manual
cross-ios universal examples
passed #686796
documentation

00:05:54