Fix GCC -Wunused-function

debug_sync_set_action(): Declare the dummy function inline,
to silence a warning about declared-but-unused static function.
This amends commit 3ccd6766.

sql/debug_sync.h

@@ -49,7 +49,8 @@ extern uchar *debug_sync_value_ptr(THD *thd);
 static inline void debug_sync_init_thread(THD *thd) {}
 static inline void debug_sync_end_thread(THD *thd) {}
 static inline void debug_sync_reset_thread(THD *thd) {}
-static bool debug_sync_set_action(THD *thd, const char *action_str, size_t len) {return false;}
+static inline bool debug_sync_set_action(THD *, const char *, size_t)
+{ return false; }
 #endif /* defined(ENABLED_DEBUG_SYNC) */
 
 #endif /* DEBUG_SYNC_INCLUDED */