gtp: provide interface to set family

Kernel now supports for TID/I_TEI to be used in both IPv4 and IPv6 GTP
tunnels. This improves dualstack support for MS/UE so GTP traffic can
be identified with the same tunnel identifier.

Update gtp-tunnel to specify the family, since a tunnel is now
identified by the following tuple [ version, identifier, family ].

Change-Id: I584d3997ffb89cd430dfda9615a4ce0ce517ab2a
diff --git a/tools/gtp-link.c b/tools/gtp-link.c
index 6979459..325a45f 100644
--- a/tools/gtp-link.c
+++ b/tools/gtp-link.c
@@ -75,6 +75,7 @@
 {
 	int fd1 = socket(family, SOCK_DGRAM, 0);
 	int fd2 = socket(family, SOCK_DGRAM, 0);
+	int one = 1;
 
 	if (fd1 < 0 || fd2 < 0)
 		return -1;