add explicit cast to avoid GCC warnings with -Wwrite-strings

Cython/Compiler/Nodes.py

@@ -5119,7 +5119,7 @@ static int __Pyx_GetVtable(PyObject *dict, void *vtabptr) {
     int result;
     PyObject *pycobj;
 
-    pycobj = PyMapping_GetItemString(dict, "__pyx_vtable__");
+    pycobj = PyMapping_GetItemString(dict, (char *)"__pyx_vtable__");
     if (!pycobj)
         goto bad;
     *(void **)vtabptr = PyCObject_AsVoidPtr(pycobj);