package/lxc: fix license

package/lxc: fix license

Commit 5a393677314dfdaa6a4bd6a847b642ae54517538 forgot to update
license which has been changed from COPYING to LICENSE.GPL2 and
LICENSE.LGPL2.1 since version 4.0.0 and
https://github.com/lxc/lxc/commit/cc73685dd09315b292968ee91a08a8f295376b35

It should be noted that COPYING describes GPLv3, see upstream answer
here: https://github.com/lxc/lxc/issues/3484

Fixes:
 - http://autobuild.buildroot.org/results/2da182d0b84e2597e135506f8166a54660c4b2f5

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>