net: explicitly include <linux/compat.h> in net/core/sock.c

The buildbot found a config where the header isn't already implicitly
pulled in, so add an explicit include as well.

Fixes: 8c918ffb

 ("net: remove compat_sock_common_{get,set}sockopt")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David S. Miller <davem@davemloft.net>