gstreamer1.0-plugins-base: Fix OpenGL setup by enabling GBM
With no window manager like X11 or Wayland available, we need to use the GBM backend in order to use OpenGL with GStreamer. Enabling it prevents a build error with gstreamer1.0-plugins-bad which expects the gstreamer-gl module to be avialable when opengl is enabled in DISTRO_FEATURES.