FAQ: Mention kernel support for MPLS in Open vSwitch 2.4.

The recent mailing list thread about MPLS reminded me that the FAQ needs an
update.

CC: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>
Acked-by: Simon Horman <horms@verge.net.au>