net: skbuff: drop the linux/sched.h include

linux/sched.h was added for skb_mstamp_* (all the way back
before linux/sched.h got split and linux/sched/clock.h created).
We don't need it in skbuff.h any more.

Sadly this change is currently a noop because linux/dma-mapping.h
and net/page_pool.h pull in half of the universe.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>