Fix a function pointer type mismatch for C23.

Conveniently, the C++ style "just don't name the argument if you're not going to use it" syntax is in C23. Inconveniently, given that this has already been by far the most troublesome C update ever, I'm assuming that the switch is likely to bounce, so this change goes with backwards-compatible syntax.

Change-Id: Ic67398042cb70c22a833b8bf8a745177a50dd9cd
diff --git a/clatd/main.c b/clatd/main.c
index f888041..7aa1671 100644
--- a/clatd/main.c
+++ b/clatd/main.c
@@ -37,7 +37,7 @@
 /* function: stop_loop
  * signal handler: stop the event loop
  */
-static void stop_loop() { running = 0; };
+static void stop_loop(__attribute__((unused)) int unused) { running = 0; };
 
 /* function: print_help
  * in case the user is running this on the command line