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
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