namespace.h: fix compilation on Linux < 4.10

namespace.h: fix compilation on Linux < 4.10

sys-utils/lsns.c: In function ‘add_namespace_from_sock’:
sys-utils/lsns.c:580:19: error: ‘SIOCGSKNS’ undeclared (first use in this function)
  nsfd = ioctl(sk, SIOCGSKNS);
                   ^~~~~~~~~

SIOCGSKNS was added in https://github.com/torvalds/linux/commit/c62cce2caee558e18aa05c01c2fd3b40f07174f2.

Fixes: https://github.com/util-linux/util-linux/issues/3502
Signed-off-by: Thomas Devoogdt <thomas.devoogdt@barco.com>
(cherry picked from commit 10ee66ed1b08586bde42582e34355418eef33ef5)