Silence an "unused" C compiler warning when we cannot easily decide whether to...

Silence an "unused" C compiler warning when we cannot easily decide whether to include the "__Pyx_Method_ClassMethod" utility code or not.

Cython/Utility/CythonFunction.c

@@ -1216,7 +1216,7 @@ static int __pyx_FusedFunction_init(void) {
 //////////////////// ClassMethod.proto ////////////////////
 
 #include "descrobject.h"
-static PyObject* __Pyx_Method_ClassMethod(PyObject *method); /*proto*/
+static CYTHON_UNUSED PyObject* __Pyx_Method_ClassMethod(PyObject *method); /*proto*/
 
 //////////////////// ClassMethod ////////////////////