avoid compiler warning on GIL-enabled build

Author: ngoldbaum

Modules/_testcapimodule.c

@@ -2419,12 +2419,15 @@ test_critical_sections(PyObject *module, PyObject *Py_UNUSED(args))
     Py_BEGIN_CRITICAL_SECTION2(module, module);
     Py_END_CRITICAL_SECTION2();
 
+#ifdef Py_GIL_DISABLED
+    // avoid unused variable compiler warning on GIL-enabled build
     PyMutex mut = {0};
     Py_BEGIN_CRITICAL_SECTION_MUTEX(&mut);
     Py_END_CRITICAL_SECTION();
 
     Py_BEGIN_CRITICAL_SECTION2_MUTEX(&mut, &mut);
     Py_END_CRITICAL_SECTION2();
+#endif
 
     Py_RETURN_NONE;
 }