tools/include: fix clean and rework COPYING for installed Xen public header

tools/include: fix clean and rework COPYING for installed Xen public header

Use actual include directory used to install the public header in
COPYING file.

Also, move the input file out of "tools/include/xen/" because that
directory is removed on `make clean`.

We can't used ./configure because $includedir contain another
variable, so the change is done in Makefile.

Fixes: 4ea75e9a9058 ("Rework COPYING installed in /usr/include/xen/, due to several licences")
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>
master commit: 25b55688e1f20ebb1bb15d0e82079e91cd787e5b
master date: 2022-12-01 16:14:59 +0100