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>