examples/gl/gtk: Add missing dependency on gstgl
Occasionally, a clean build would fail like this:
In file included from ../subprojects/gst-plugins-base/tests/examples/gl/gtk/gstgtk.c:24:
../subprojects/gst-plugins-base/gst-libs/gst/gl/gl.h:25:10: fatal error: gst/gl/gl-enumtypes.h: No such file or directory
25 | #include <gst/gl/gl-enumtypes.h>
| ^~~~~~~~~~~~~~~~~~~~~~~
Add the missing dependency so that the headers are generated beforehand.