Tilemap/VX: Ensure proxy objects don't outlive their parents

Either of these would previously crash (same for VX):

tm = Tilemap.new
at = tm.autotiles
tm = nil
GC.start
at[0] = Bitmap.new(1, 1)

tm = Tilemap.new
at = tm.autotiles
tm.dispose
at[0] = Bitmap.new(1, 1)

Funnily, this makes RMXP itself crash too, but crashing is
never acceptable except for possibly resource exhaustion.

binding-mruby/binding-util.h

@@ -42,6 +42,7 @@ enum CommonSymbol
 	CStone,
 	CSrect,
 	CSsrc_rect,
+	CStilemap,
 	CStileset,
 	CSautotiles,
 	CSmap_data,