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

It used to be necessary for skb_mstamp_* static inlines,
but those are gone since we moved to usec timestamps in
TCP, in 2017.

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