if_ether.h: add IEEE 802.21 Ethertype

Add the Ethertype for IEEE Std 802.21 - Media Independent Handover
Protocol. This Ethertype is used for network control messages.

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>