iptables: add noreturn attribute to exit_tryhelp()

Found by gcc -Wmissing-noreturn.

Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>
diff --git a/iptables.c b/iptables.c
index 25bc8cc..19c2af5 100644
--- a/iptables.c
+++ b/iptables.c
@@ -237,7 +237,7 @@
 	IPT_DOTTED_MASK
 };
 
-static void
+static void __attribute__((noreturn))
 exit_tryhelp(int status)
 {
 	if (line != -1)