BSSGP: cosmetic use OSMO_IMSI_BUF_SIZE from libosmocore

Change-Id: I2079118100a3422e9c2ee63a5ac74e3ee81080b3
diff --git a/src/gprs_bssgp_pcu.cpp b/src/gprs_bssgp_pcu.cpp
index 0be3b3d..efddc03 100644
--- a/src/gprs_bssgp_pcu.cpp
+++ b/src/gprs_bssgp_pcu.cpp
@@ -188,7 +188,7 @@
 
 static int gprs_bssgp_pcu_rx_paging_ps(struct msgb *msg, struct tlv_parsed *tp)
 {
-	char imsi[16];
+	char imsi[OSMO_IMSI_BUF_SIZE];
 	uint16_t pgroup;
 	const uint8_t *mi;
 	uint8_t mi_len;