Cleanup: move RNA utility functions into a generic module

Avoid having to include bpy_rna.h for enum utility functions,
recently added to idprop_py_ui_api.c.

source/blender/python/intern/bpy_msgbus.c

@@ -21,6 +21,7 @@
 
 #include <Python.h>
 
+#include "../generic/py_capi_rna.h"
 #include "../generic/py_capi_utils.h"
 #include "../generic/python_utildefines.h"
 #include "../mathutils/mathutils.h"