Add mock-up function for unit tests

They failed without it.

test/birdtest.c

@@ -487,6 +487,7 @@ void cmd_reconfig(char *name UNUSED, int type UNUSED, int timeout UNUSED) {}
 void cmd_reconfig_confirm(void) {}
 void cmd_reconfig_undo(void) {}
 void cmd_reconfig_status(void) {}
+void cmd_graceful_restart(void) {}
 void cmd_shutdown(void) {}
 void cmd_reconfig_undo_notify(void) {}