tests: Mark tftpd timer function as noreturn

This avoids the below compiler warning:

tftpd.c:280:1: warning: function 'timer' could be declared with
    attribute 'noreturn' [-Wmissing-noreturn]

Closes: #13534
Reviewed-by: Daniel Stenberg <daniel@haxx.se>

tests/server/tftpd.c

@@ -276,6 +276,9 @@ static void mysignal(int sig, void (*handler)(int))
   sigaction(sig, &sa, NULL);
 }
 
+#ifdef HAVE_SIGSETJMP
+CURL_NORETURN
+#endif
 static void timer(int signum)
 {
   (void)signum;