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>

include/linux/skbuff.h

@@ -23,7 +23,6 @@
 #include <linux/atomic.h>
 #include <asm/types.h>
 #include <linux/spinlock.h>
-#include <linux/net.h>
 #include <linux/textsearch.h>
 #include <net/checksum.h>
 #include <linux/rcupdate.h>