mpls: Fix the openvswitch select of NET_MPLS_GSO

Fix the OPENVSWITCH Kconfig option and old Kconfigs by having
OPENVSWITCH select both NET_MPLS_GSO and MPLSO.

A Kbuild test robot reported that when NET_MPLS_GSO is selected by
OPENVSWITCH the generated .config is broken because MPLS is not
selected.

Cc: Simon Horman <horms@verge.net.au>
Fixes: cec9166c mpls: Refactor how the mpls module is built
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Reviewed-by: Simon Horman <horms@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>