[VETH]: move veth.h to include/linux

Move veth.h from net/ to linux/ since it is a user api, and add it to
user header processing Kbuild.

[ Use header-y as suggested by Sam Ravnborg.  -DaveM ]

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>