drm-uapi: Add sync_file.h

Based on <linux/sync_file.h> with BSD portability conditional.
At least FreeBSD supports SYNC_IOC_* via LinuxKPI in DRM.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5463>
1 file changed