Fix format string error (string needs %s)

In a49475b5a822773ce10e28bf5a02e91de78bd063 we introduce the use of
bssgp_pdu_str() and change from printing the numeric code to the
stringified version of the message code.  However, the format string
was not updated accordingly :/

Change-Id: I7173b692fb1f222aab44cd4f44a482038d0f51dc
Fixes: Coverity CID 169684
diff --git a/src/gprs_bssgp_pcu.cpp b/src/gprs_bssgp_pcu.cpp
index e73a8c0..aeb0942 100644
--- a/src/gprs_bssgp_pcu.cpp
+++ b/src/gprs_bssgp_pcu.cpp
@@ -248,7 +248,8 @@
 		LOGP(DBSSGP, LOGL_DEBUG, "rx BSSGP_PDUT_FLOW_CONTROL_MS_ACK\n");
 		break;
 	default:
-		LOGP(DBSSGP, LOGL_NOTICE, "BSSGP BVCI=%u PDU type 0x%02x unknown\n", bctx->bvci, bssgp_pdu_str(pdu_type));
+		LOGP(DBSSGP, LOGL_NOTICE, "BSSGP BVCI=%u PDU type %s unknown\n",
+			bctx->bvci, bssgp_pdu_str(pdu_type));
 		rc = bssgp_tx_status(BSSGP_CAUSE_PROTO_ERR_UNSPEC, NULL, msg);
 		break;
 	}