net: Add header for usage of fls64()

[ Upstream commit 8681ef1f3d295bd3600315325f3b3396d76d02f6 ]

Fixes: 3b89ea9c5902 ("net: Fix for_each_netdev_feature on Big endian")
Suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>