string_to_ip: constify "s" arg

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/lib/net_utils.c b/lib/net_utils.c
index f03b098..b425a68 100644
--- a/lib/net_utils.c
+++ b/lib/net_utils.c
@@ -28,7 +28,7 @@
 
 #include <common.h>
 
-IPaddr_t string_to_ip(char *s)
+IPaddr_t string_to_ip(const char *s)
 {
 	IPaddr_t addr;
 	char *e;