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

splice.h is included since commit a60e3cc7 ("net: make
skb_splice_bits more configureable") but really even then
all we needed is some forward declarations. Most of that
code is now gone, and remaining has fwd declarations.

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