UI/Physics: Show error enabling Rigid Body if compiled without Bullet

Would just silently fail, which is confusing.
Should only impact custom builds.

source/blender/blenkernel/intern/rigidbody.c

@@ -2116,6 +2116,7 @@ void BKE_rigidbody_ensure_local_object(Main *bmain, Object *ob)
 
 bool BKE_rigidbody_add_object(Main *bmain, Scene *scene, Object *ob, int type, ReportList *reports)
 {
+  BKE_report(reports, RPT_ERROR, "Compiled without Bullet physics engine");
   return false;
 }