qapi: Ignore files created during make check

After an in-tree build and run of 'make check-{qapi-schema,unit}',
I noticed some leftover files.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Wenchao Xia <wenchaoqemu@gmail.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
