In commit e2bbc4ea we changed the QAPI modules to name the built-in
module "./builtin" rather than None, but forgot to update the Sphinx
plugin. The effect of this was that when the plugin generated a dependency
file it was including a bogus dependency on a non-existent file named
"builtin", which meant that ninja would run Sphinx and rebuild all
the documentation every time even if nothing had changed.

Update the plugin to use the new name of the builtin module.

Fixes: e2bbc4ea
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Tested-by: Eric Blake <eblake@redhat.com>
Message-id: 20210212161311.28915-1-peter.maydell@linaro.org