We commonly define the header guard symbol without an explicit value.
Normalize the exceptions.

Done with scripts/clean-header-guards.pl.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190315145123.28030-8-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>