Move pdp_get_peer_ipv() to lib/util.*

Preparation for next commit, where this function will be needed inside
libmisc (lib/*).

Change-Id: Ibab4f6c09d1e5f0e9cfaea28ae1e7ab5b5c219b5
diff --git a/ggsn/ggsn_vty.c b/ggsn/ggsn_vty.c
index 0a86f49..b85df77 100644
--- a/ggsn/ggsn_vty.c
+++ b/ggsn/ggsn_vty.c
@@ -37,6 +37,8 @@
 #include "../gtp/gtp.h"
 #include "../gtp/pdp.h"
 
+#include "../lib/util.h"
+
 #include "ggsn.h"
 
 #define PREFIX_STR	"Prefix (Network/Netmask)\n"