netfilter: xt_LOG: do print MAC header on FORWARD

I am observing consistent behavior even with bridges, so let's unlock
this. xt_mac is already usable in FORWARD, too. Section 9 of
http://ebtables.sourceforge.net/br_fw_ia/br_fw_ia.html#section9

 says
the MAC source address is changed, but my observation does not match
that claim -- the MAC header is retained.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
[Patrick; code inspection seems to confirm this]
Signed-off-by: Patrick McHardy <kaber@trash.net>