if_ether.h: Add IEEE 802.1 Local Experimental Ethertype 1.

Add EthType 0x88b5.
This Ethertype value is available for public use for prototype and
vendor-specific protocol development,as defined in Amendment 802a
to IEEE Std 802.

Signed-off-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>