mpls: Fix config check for mpls.

Fixes MPLS GSO for case when mpls is compiled as kernel module.

Fixes: 0d89d203 ("MPLS: Add limited GSO support").
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -2522,7 +2522,7 @@ static int illegal_highdma(struct net_device *dev, struct sk_buff *skb)
 /* If MPLS offload request, verify we are testing hardware MPLS features
  * instead of standard features for the netdev.
  */
-#ifdef CONFIG_NET_MPLS_GSO
+#if IS_ENABLED(CONFIG_NET_MPLS_GSO)
 static netdev_features_t net_mpls_features(struct sk_buff *skb,
 					   netdev_features_t features,
 					   __be16 type)