mpls: move mpls_hdr to a common location

This will be also used by openvswitch.

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Acked-by: David Ahern <dsa@cumulusnetworks.com>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>