use named variant when allocating msgb's

when we generate a talloc report (SIGUSR1), we can now see which system
allocated a given msgb, this helps memory leak debugging
diff --git a/openbsc/src/rs232.c b/openbsc/src/rs232.c
index 2a64de5..a584723 100644
--- a/openbsc/src/rs232.c
+++ b/openbsc/src/rs232.c
@@ -127,7 +127,7 @@
 	int rc = 0;
 
 	if (!sh->rx_msg) {
-		sh->rx_msg = msgb_alloc(SERIAL_ALLOC_SIZE);
+		sh->rx_msg = msgb_alloc(SERIAL_ALLOC_SIZE, "RS232 Rx");
 		sh->rx_msg->l2h = NULL;
 		sh->rx_msg->trx = sh->bts->c0;
 	}