Bluetooth: Host: ATT: Fix build warning with clang

Fix the following warning
att.c:734:3: warning: label followed by a declaration is a C23 extension
[-Wc23-extensions]
  734 |                 k_tid_t current_thread = k_current_get();
      |                 ^
By wrapping that code as a compound statement

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>

Author: aescolar

subsys/bluetooth/host/att.c

@@ -729,7 +729,7 @@ static struct net_buf *bt_att_chan_create_pdu(struct bt_att_chan *chan, uint8_t
 		/* Use a timeout only when responding/confirming */
 		timeout = BT_ATT_TIMEOUT;
 		break;
-	default:
+	default: {
 		k_tid_t current_thread = k_current_get();
 
 		if (current_thread == k_work_queue_thread_get(&k_sys_work_q)) {
@@ -742,6 +742,7 @@ static struct net_buf *bt_att_chan_create_pdu(struct bt_att_chan *chan, uint8_t
 			timeout = K_FOREVER;
 		}
 	}
+	}
 
 	/* This will reserve headspace for lower layers */
 	buf = bt_l2cap_create_pdu_timeout(&att_pool, 0, timeout);