gh-90949: Fix an "unused function" compiler warning introduced in GH-139234 (#139558)

Fix a compiler warning `-Wunused-function` after f04bea44c3.

The `set_invalid_arg` function in `Modules/pyexpat.c` may be unused if the underlying Expat
version is less than 2.4.0.

Modules/pyexpat.c

@@ -198,12 +198,14 @@ set_error(pyexpat_state *state, xmlparseobject *self, enum XML_Error code)
     return NULL;
 }
 
+#if XML_COMBINED_VERSION >= 20400
 static PyObject *
 set_invalid_arg(pyexpat_state *state, xmlparseobject *self, const char *errmsg)
 {
     SET_XML_ERROR(state, self, XML_ERROR_INVALID_ARGUMENT, errmsg);
     return NULL;
 }
+#endif
 
 #undef SET_XML_ERROR