Fix muldef in protobuf pyext.
The other definition is in python/google/protobuf/internal/python_protobuf.cc.
Note: mandatory check (NEED_CHECK) was skipped
@@ -2882,8 +2882,8 @@ PyTypeObject CMessage_Type = {
// --- Exposing the C proto living inside Python proto to C code:
-const Message* (*GetCProtoInsidePyProtoPtr)(PyObject* msg);
-Message* (*MutableCProtoInsidePyProtoPtr)(PyObject* msg);
+extern const Message* (*GetCProtoInsidePyProtoPtr)(PyObject* msg);
+extern Message* (*MutableCProtoInsidePyProtoPtr)(PyObject* msg);
static const Message* GetCProtoInsidePyProtoImpl(PyObject* msg) {
if (!PyObject_TypeCheck(msg, &CMessage_Type)) {
