Properly format IMSI before using it in trap

For some reason Max' commits introducing the CTRL/trap interface
about one year ago didn't convert the IMSI to its actual textual
representation before usign it in the CTRL interface.

Let's clean that up by properly interpreting the IMSI.

Change-Id: I8b20d2e47a29de266d93a7ddd5e6877f7e346a63
diff --git a/ggsn/ggsn.c b/ggsn/ggsn.c
index 05a56ae..c6a6dac 100644
--- a/ggsn/ggsn.c
+++ b/ggsn/ggsn.c
@@ -146,7 +146,7 @@
 
 	const char *addrstr = in46a_ntop(&member->addr, addrbuf, sizeof(addrbuf));
 
-	snprintf(val, sizeof(val), "%" PRIu64 ",%s", pdp->imsi, addrstr);
+	snprintf(val, sizeof(val), "%s,%s", imsi_gtp2str(&pdp->imsi), addrstr);
 
 	if (ctrl_cmd_send_trap(gsn->ctrl, var, val) < 0) {
 		LOGP(DGGSN, LOGL_ERROR, "Failed to create and send TRAP for IMSI %" PRIu64 " [%s].\n", pdp->imsi, var);