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

It appears nothing needs it. The kernel builds fine with this
include removed, building an otherwise empty source file with:

 #include <linux/skbuff.h>
 #ifdef _LINUX_NET_H
 #error linux/net.h is back
 #endif

works too (meaning net.h is not just pulled in indirectly).

This gives us a slight 0.5% reduction in the pre-processed size
of skbuff.h.

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