6lowpan: move 6lowpan header to include/net

This header is used by bluetooth and ieee802154 branch. This patch
move this header to the include/net directory to avoid a use of a
relative path in include.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc include/net/6lowpan.h
Simple merge

diff --cc net/bluetooth/6lowpan.c
Simple merge

diff --cc net/ieee802154/6lowpan_iphc.c
Simple merge

diff --cc net/ieee802154/6lowpan_rtnl.c
Simple merge

diff --cc net/ieee802154/reassembly.c
Simple merge