sgsnemu: Implement ping on IPv6 APNs

Related: OS#4434
Change-Id: If9ca7c37a1a397bbc3f8912d67bccdabc4968e0c
diff --git a/lib/icmpv6.c b/lib/icmpv6.c
index 1bddf65..ac1474d 100644
--- a/lib/icmpv6.c
+++ b/lib/icmpv6.c
@@ -42,7 +42,7 @@
 };
 
 /* Prepends the ipv6 header and returns checksum content */
-static uint16_t icmpv6_prepend_ip6hdr(struct msgb *msg, const struct in6_addr *saddr,
+uint16_t icmpv6_prepend_ip6hdr(struct msgb *msg, const struct in6_addr *saddr,
 				  const struct in6_addr *daddr)
 {
 	uint32_t len;