string_to_ip: constify "s" arg

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/include/net.h b/include/net.h
index 5e3495b..95ef8ab 100644
--- a/include/net.h
+++ b/include/net.h
@@ -514,7 +514,7 @@
 extern void	ip_to_string (IPaddr_t x, char *s);
 
 /* Convert a string to ip address */
-extern IPaddr_t string_to_ip(char *s);
+extern IPaddr_t string_to_ip(const char *s);
 
 /* Convert a VLAN id to a string */
 extern void	VLAN_to_string (ushort x, char *s);