qapi: Fix C code generation for 'if'

When commit 5d83b9a1 "qapi: replace if condition list with dict
{'all': [...]}" made cgen_ifcond() and docgen_ifcond() recursive, it
messed up parenthesises in the former, and got them right in the
latter, as the previous commit demonstrates.

To fix, adopt the latter's working code for the former.  This
generates the correct code from the previous commit's commit message.

Fixes: 5d83b9a1
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210831123809.1107782-6-armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>