qapi: Fix a botched type annotation

qapi: Fix a botched type annotation

Mypy is unhappy:

    $ mypy --config-file=scripts/qapi/mypy.ini `git-ls-files scripts/qapi/\*py`
    scripts/qapi/common.py:208: error: Function is missing a return type annotation
    scripts/qapi/common.py:227: error: Returning Any from function declared to return "str"

Messed up in commit ccea6a8637 "qapi: Factor common recursion out of
cgen_ifcond(), docgen_ifcond()".  Tidy up.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210908045428.2689093-2-armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>