UAPI: add MPLS label stack definition

Labels for the Multiprotocol Label Switching are defined in RFC 3032
which was superseded by RFC 5462. Add the definition to UAPI and a stub
header for include/linux.

Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Mathias Kretschmer <mathias.kretschmer@fokus.fraunhofer.de>
Signed-off-by: David S. Miller <davem@davemloft.net>