bankd_main: don't use fprintf if we can use osmocom logging API

Change-Id: I0b6ed933ffb3e3d689b344bd215357577e3120aa
diff --git a/src/bankd_main.c b/src/bankd_main.c
index e3ec23f..3a65629 100644
--- a/src/bankd_main.c
+++ b/src/bankd_main.c
@@ -199,7 +199,8 @@
 		server_conn_send_rspro(srvc, resp);
 		break;
 	default:
-		fprintf(stderr, "Unknown/Unsupported RSPRO PDU type: %u\n", pdu->msg.present);
+		LOGPFSML(srvc->fi, LOGL_ERROR, "Unknown/Unsupported RSPRO PDU type: %u\n",
+			 pdu->msg.present);
 		return -1;
 	}