xfce-polkit: fix warning not able to copy license

COPYING is a link on LICENSE. It seems that cause the warning so use LICENSE.
Fixes:
| WARNING: xfce-polkit-0.2-r0 do_populate_lic: Could not copy license file /home/superandy/tmp/oe-core-glibc/work/cortexa7t2hf-neon-vfpv4-angstrom-linux-gnueabi/xfce-polkit/0.2-r0/git/COPYING to /home/superandy/tmp/oe-core-glibc/work/cortexa7t2hf-neon-vfpv4-angstrom-linux-gnueabi/xfce-polkit/0.2-r0/license-destdir/xfce-polkit/COPYING: [Errno 2] No such file or directory: '/home/superandy/tmp/oe-core-glibc/work/cortexa7t2hf-neon-vfpv4-angstrom-linux-gnueabi/xfce-polkit/0.2-r0/license-destdir/xfce-polkit/COPYING'

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Armin Kuster <akuster808@gmail.com>