iris: Include linux/sync_file.h instead of cut and pasting contents

iris: Include linux/sync_file.h instead of cut and pasting contents

Lets us drop some cut and pasted kernel header contents.

Linux 4.7 came out 4 years before we the first officially supported
release of this driver; iris won't run on kernels older than 4.16,
and 4.18.11+ is strongly recommended.  So I suspect it's safe to
assume that a kernel header from 4.7 will exist at build time.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3802>