undef the COMMON_FIELDS macro in funcobject.h after it is used

Summary:
based on D47485335

avoid the macro leaking to C++ code that happens to use the name `COMMON_FIELDS`

Reviewed By: zsol

Differential Revision: D56454529

fbshipit-source-id: 5b7192abf84748fb65548db3f350d375652fd5a7

Include/cpython/funcobject.h

@@ -60,6 +60,10 @@ typedef struct {
      */
 } PyFunctionObject;
 
+// START META PATCH (undef COMMON_FIELDS to avoid leaking into C++ code)
+#undef COMMON_FIELDS
+// END META PATCH
+
 PyAPI_DATA(PyTypeObject) PyFunction_Type;
 
 #define PyFunction_Check(op) Py_IS_TYPE((op), &PyFunction_Type)