sockets: Fix compilation on older kernel headers

Since kernel commit 9c501935a3 the sys/socket.h
should be included prior the linux/netlink.h.

Make it so.
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

sockets.c

+#include <sys/socket.h>
 #include <linux/netlink.h>
 #include <linux/types.h>
 #include <linux/net.h>