docs/xen-headers: allow headers to be symlinks

docs/xen-headers: allow headers to be symlinks

There's no apparent reason not to permit this, and since we don't
support out-of-source-tree builds, the least overhead way of doing
multiple, differently configured (perhaps different architecture)
builds from a single source tree is to create symlinked build trees.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>