configure.ac: add lsns option

configure.ac: add lsns option

lsns unconditionally uses NS_GET_NSTYPE since version 2.38 and
https://git.kernel.org/pub/scm/utils/util-linux/util-linux.git/commit/?id=de72df79d72fa906e71e2ac922d8745ff22deee5
which is only available since kernel 4.11 and
https://github.com/torvalds/linux/commit/e5ff5ce6e20ee22511398bb31fb912466cf82a36
resulting in the following build failure:

sys-utils/lsns.c: In function 'add_namespace_for_nsfd':
sys-utils/lsns.c:719:25: error: 'NS_GET_NSTYPE' undeclared (first use in this function)
  719 |  clone_type = ioctl(fd, NS_GET_NSTYPE);
      |                         ^~~~~~~~~~~~~

So add an option to allow the user to disable lsns

Fixes:
 - http://autobuild.buildroot.org/results/572ecf8e37ac733a4e4265f4f78f35230337278e

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>