packet: fix broken build.

This patch fixes a broken build due to a missing header:
...
  CC      net/ipv4/proc.o
In file included from include/net/net_namespace.h:15,
                 from net/ipv4/proc.c:35:
include/net/netns/packet.h:11: error: field 'sklist_lock' has incomplete type
...

The lock of netns_packet has been replaced by a recent patch to be a mutex instead of a spinlock,
but we need to replace the header file to be linux/mutex.h instead of linux/spinlock.h as well.

See commit 0fa7fa98:
packet: Protect packet sk list with mutex (v2) patch,

Signed-off-by: Rami Rosen <rosenr@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>