Make IP address helper accessible via header file

Change-Id: I4bc157bf296e28678de6d9c9823f91810132a58c
diff --git a/include/osmocom/bsc/abis_rsl.h b/include/osmocom/bsc/abis_rsl.h
index 886e7d6..ba44669 100644
--- a/include/osmocom/bsc/abis_rsl.h
+++ b/include/osmocom/bsc/abis_rsl.h
@@ -35,6 +35,8 @@
 
 #define GSM48_LEN2PLEN(a)	(((a) << 2) | 1)
 
+const char *ip_to_a(uint32_t ip);
+
 int rsl_bcch_info(const struct gsm_bts_trx *trx, enum osmo_sysinfo_type si_type, const uint8_t *data, int len);
 int rsl_sacch_filling(struct gsm_bts_trx *trx, uint8_t type,
 		      const uint8_t *data, int len);