MDEV-26533 fixup: GCC -Wformat

sql/signal_handler.cc

 /* Copyright (c) 2011, 2012, Oracle and/or its affiliates.
-   Copyright (c) 2011, 2014, SkySQL Ab.
+   Copyright (c) 2011, 2021, MariaDB Corporation.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -183,7 +183,7 @@ extern "C" sig_handler handle_fatal_signal(int sig)
                         max_used_connections);
 
   if (thread_scheduler)
-    my_safe_printf_stderr("max_threads=%u\n",
+    my_safe_printf_stderr("max_threads=%lu\n",
                           thread_scheduler->max_threads +
                           extra_max_connections);