mpls: Allow mpls_gso to be built as module

Kconfig already allows mpls to be built as module. Following patch
fixes Makefile to do same.

CC: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>