GTP: Replace recently introduced imsi_str2gtp()

Replace with the version from osmo-sgsn, renamed so
as not to collide with that version.

Change-Id: I910d5339a823332277ce7b5854d5c943ed69ea81
diff --git a/ggsn/ggsn_vty.c b/ggsn/ggsn_vty.c
index fd2c6ad..f76018e 100644
--- a/ggsn/ggsn_vty.c
+++ b/ggsn/ggsn_vty.c
@@ -899,7 +899,7 @@
 		return CMD_WARNING;
 	}
 
-	imsi = imsi_str2gtp(argv[1]);
+	imsi = gtp_imsi_str2gtp(argv[1]);
 
 	if (argc > 2) {
 		nsapi = atoi(argv[2]);