windowed-output-api.h: Do an explicit cast to avoid C++ build failure

Address the following build issue:

error: invalid conversion from ‘const void*’ to ‘const
weston_windowed_output_api*’ [-fpermissive] return
weston_plugin_api_get(compositor, api_names[type],
~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
const void* sizeof(struct weston_windowed_output_api));

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>