examples/gl/qt: Don't include GL/glx.h unconditionally

The header file might not be available on non-*nix platform.

Part-of: <gstreamer/gst-plugins-base!740>
33 jobs for !740 with gl-qt-example-build in 32 minutes and 1 second (queued for 5 seconds)
detached
Status Job ID Name Coverage
  Build Docker
passed #3493747
alpine amd64 manifest builder docker

00:00:27

passed #3493746
android docker

00:00:18

passed #3493744
cerbero fedora amd64 docker

00:00:29

passed #3493743
fedora amd64 docker

00:00:19

passed #3493749
gst-indent amd64 docker

00:00:19

 
  Preparation
passed #3493752
gst indent

00:00:19

passed #3493751
manifest

00:00:26

 
  Build
passed #3493771
gstreamer
build cerbero cross-android universal

00:10:47

passed #3493775
gst-ios-13.2
build cerbero cross-ios universal

00:29:48

passed #3493772
gstreamer
build cerbero cross win32

00:08:14

passed #3493773
gstreamer
build cerbero cross win64

00:07:42

passed #3493768
gstreamer
build cerbero fedora x86_64

00:06:12

passed #3493774
gst-macos-10.15
build cerbero macos x86_64

00:08:53

passed #3493759
build clang fedora x86_64

00:04:33

manual #3493766
docker windows 1809 allowed to fail manual
build msys2
passed #3493754
build nodebug fedora x86_64

00:04:25

passed #3493756
build static fedora x86_64

00:04:26

passed #3493757
build static nodebug fedora x86_64

00:03:34

passed #3493761
docker windows 1809
build vs2017 amd64

00:11:42

passed #3493763
docker windows 1809
build vs2017 x86

00:12:00

 
  Test
passed #3493776
gstreamer
check fedora

00:06:30

passed #3493777
gstreamer
integration testsuites fedora 1/4

00:15:36

passed #3493778
gstreamer
integration testsuites fedora 2/4

00:15:02

passed #3493779
gstreamer
integration testsuites fedora 3/4

00:14:31

passed #3493780
gstreamer
integration testsuites fedora 4/4

00:14:34

passed #3493784
gstreamer
valgrind bad

00:05:47

passed #3493781
gstreamer
valgrind base

00:06:03

passed #3493785
gstreamer
valgrind ges

00:14:43

passed #3493782
gstreamer
valgrind good

00:08:30

passed #3493783
gstreamer
valgrind ugly

00:03:36

 
  Integrate
passed #3493786
cross-android universal examples

00:05:37

passed #3493788
gst-ios-13.2
cross-ios universal examples

00:01:14

passed #3493787
documentation

00:07:43