Fix a typo in the dependency list for the manpages built from the
'interop' manual, which meant we were accidentally not including
the .hx file in the dependency list.

Fixes: e13c59fa
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200213175647.17628-4-peter.maydell@linaro.org>
Signed-off-by: Markus Armbruster <armbru@redhat.com>