netlink: include sys/socket.h

It's workaround for RHEL6, it contains old headers.

In file included from libnetlink.c:2:
/usr/include/linux/netlink.h:34: error: expected specifier-qualifier-list before ‘sa_family_t’
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Acked-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>

libnetlink.c

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