Fix linkage of _PyTestCapi_Init_Vectorcall

Author: arhadthedev

Modules/_testcapi/vectorcall.c

@@ -229,7 +229,7 @@ static PyTypeObject MethodDescriptor2_Type = {
 };
 
 
-int
+PyAPI_FUNC(int) 
 _PyTestCapi_Init_Vectorcall(PyObject *m) {
     if (PyModule_AddFunctions(m, TestMethods) < 0) {
         return -1;