net: skbuff.h: drop unneeded <linux/slab.h>

<linux/skbuff.h> does not use nor need <linux/slab.h>, so drop this
header file from skbuff.h.

<linux/skbuff.h> is currently #included in around 1200 C source and
header files, making it the 31st most-used header file.

Build tested [allmodconfig] on 20 arch-es.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>