Docs: add GL_CFLAGS to GTK_DOC_CFLAGS

Otherwise introspection may fail because wayland-client.h cannot be found.
17 jobs for 1.16 in 56 minutes and 3 seconds (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed #367605
gst indent

00:00:31

passed #367604
manifest

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  Build
passed #367606
build fedora x86_64

00:04:32

passed #367607
build nodebug fedora x86_64

00:07:15

 
  Test
passed #367608
check fedora

00:04:02

passed #367609
integration testsuites fedora

00:08:42

 
  Full Builds
failed #530958
build cerbero android universal

passed #367612
build cerbero cross win32

00:39:21

passed #367613
build cerbero cross win64

00:37:22

passed #367610
build cerbero fedora x86_64

00:37:07

failed #367611
build cerbero android universal

01:00:01

 
  Full Tests
skipped #367617
valgrind bad
skipped #367614
valgrind base
skipped #367618
valgrind ges
skipped #367615
valgrind good
skipped #367616
valgrind ugly
 
  Apps
skipped #367619
android universal examples
 
Name Stage Failure
failed
build cerbero android universal Full Builds There has been a missing dependency failure
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