meson: Add gtk guard
Add gtk guard when building test-vaapicontext
../subprojects/gstreamer-vaapi/tests/elements/test-vaapicontext.c:26:10: fatal error: gtk/gtk.h: No such file or directory
Due to an influx of spam, we have had to impose restrictions on new accounts. Please see this wiki page for instructions on how to get full permissions. Sorry for the inconvenience.
Add gtk guard when building test-vaapicontext
../subprojects/gstreamer-vaapi/tests/elements/test-vaapicontext.c:26:10: fatal error: gtk/gtk.h: No such file or directory