meson: Add gstreamer-gl-1.0 pkgconfig vars to internal dependency
So we can fetch gl_winsys, gl_platforms, etc directly using gst_gl_dep.get_variable() when building gstreamer as a subproject, or when building gst-plugins-rs's gtk4 plugin.
So we can fetch gl_winsys, gl_platforms, etc directly using gst_gl_dep.get_variable() when building gstreamer as a subproject, or when building gst-plugins-rs's gtk4 plugin.