crtc-kms: Include MetaCrtc in MetaCrtcKms struct

Otherwise it can't be a GObject inheriting MetaCrtc. This fixes warning
logged as

"invalid uninstantiatable type '(null)' in cast to 'MetaCrtcKms'"

Fixes: 1ce9e379d9

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1308

src/backends/native/meta-crtc-kms.c

@@ -37,6 +37,8 @@
 
 struct _MetaCrtcKms
 {
+  MetaCrtc parent;
+
   MetaKmsCrtc *kms_crtc;
 
   MetaKmsPlane *primary_plane;