ceval.c: mark debug 'filename' variable as unused (GH-11074)

Use GCC __attribute__((unused)) to mark the debug variable 'filename'
as unused in Python/ceval.c.

Python/ceval.c

@@ -824,8 +824,12 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
     PyObject *consts;
 #if defined(Py_DEBUG) || defined(LLTRACE)
     /* Make it easier to find out where we are with a debugger */
+#ifdef __GNUC__
+    char *filename __attribute__((unused));
+#else
     char *filename;
 #endif
+#endif
 
 /* Tuple access macros */