qapi: Add 'if' to implicit struct members

The generated code is for now *unconditional*.  Later patches generate
the conditionals.

Note that union discriminators may not have 'if' conditionals.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20181213123724.4866-14-marcandre.lureau@redhat.com>
Message-Id: <20181213123724.4866-15-marcandre.lureau@redhat.com>
[Patches squashed, commit message tweaked]
Signed-off-by: Markus Armbruster <armbru@redhat.com>