clutter/keyval: Fix keyval_name() transfer annotation

clutter_keyval_name() returns a pointer to a static array, not
newly allocated memory. Add a transfer annotation to indicate
that callers must not free the returned memory.

While add it, make the return value const to stress further that
callers shouldn't touch the returned memory.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3386>

clutter/clutter/clutter-keyval.c

@@ -176,9 +176,9 @@ clutter_keys_keyval_compare (const void *pkey, const void *pbase)
  * clutter_keyval_name:
  * @keyval: A key value.
  *  
- * Returns: (nullable): The corresponding symbolic name.
+ * Returns: (nullable) (transfer none): The corresponding symbolic name.
  */
-char *
+const char *
 clutter_keyval_name (unsigned int keyval)
 {
   static char buf[100];

clutter/clutter/clutter-keyval.h

@@ -35,6 +35,6 @@ void clutter_keyval_convert_case (unsigned int  symbol,
                                   unsigned int *upper);
 
 CLUTTER_EXPORT
-char * clutter_keyval_name       (unsigned int keyval);
+const char * clutter_keyval_name (unsigned int keyval);
 
 G_END_DECLS