tools/include: drop leading underscore from xen_list header

tools/include: drop leading underscore from xen_list header

A leading underscore is used to indicate auto generated headers, and
the clean use of 'rm -f _*.h' will remove those.  _xen_list.h also
uses a leading underscore, but is checked in the repo and as such
cannot be removed as part of the clean rule.

Fix this by dropping the leading underscore, so that the header is not
removed.

Fixes: a03b3552d4 ('libs,tools/include: Clean "clean" targets')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>