Use relative instead of absolute filenames in the C-level tracebacks.

This prevents traceback prints pointing to files in this way:

  File "\loewis\25\python\Modules\_ctypes\callbacks.c", line 206, in 'calling callback function'

Modules/_ctypes/callbacks.c

@@ -201,7 +201,7 @@ static void _CallPythonObject(void *mem,
 	}
 
 #define CHECK(what, x) \
-if (x == NULL) _AddTraceback(what, __FILE__, __LINE__ - 1), PyErr_Print()
+if (x == NULL) _AddTraceback(what, "_ctypes/callbacks.c", __LINE__ - 1), PyErr_Print()
 
 	result = PyObject_CallObject(callable, arglist);
 	CHECK("'calling callback function'", result);

Modules/_ctypes/callproc.c

@@ -766,7 +766,7 @@ static PyObject *GetResult(PyObject *restype, void *result, PyObject *checker)
 
 	v = PyObject_CallFunctionObjArgs(checker, retval, NULL);
 	if (v == NULL)
-		_AddTraceback("GetResult", __FILE__, __LINE__-2);
+		_AddTraceback("GetResult", "_ctypes/callproc.c", __LINE__-2);
 	Py_DECREF(retval);
 	return v;
 }