'setns.h' is not an lxc interface.

Also, a side effect of this declaration of 'setns.h' in the lxc header
files is to be distributed in the lxc tarball. This breaks the
lxc-attach command because the file is not automatically generated at
compile time.
Signed-off-by: Cedric Le Goater <clg@fr.ibm.com>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>