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/gtp/gtp.h b/gtp/gtp.h
index fd138cc..8f13ed2 100644
--- a/gtp/gtp.h
+++ b/gtp/gtp.h
@@ -402,5 +402,6 @@
 extern int eua2ipv4(struct in_addr *dst, struct ul66_t *eua);
 extern int gsna2in_addr(struct in_addr *dst, struct ul16_t *gsna);
 extern int in_addr2gsna(struct ul16_t *gsna, struct in_addr *src);
+extern const char *imsi_gtp2str(const uint64_t *imsi);
 
 #endif /* !_GTP_H */