uapi: add missing install of sync_file.h

uapi: add missing install of sync_file.h

As part of the sync framework destaging, the sync_file.h header
was moved, but an entry was not added on Kbuild to install it.
This patch resolves this omission so that "make headers_install"
installs this header.

Fixes: 460bfc41fd52 ("dma-buf/sync_file: de-stage sync_file headers")
Reported-by: Michael Ellerman <[email protected]>
Reviewed-by: Gustavo Padovan <[email protected]>
Signed-off-by: Emilio López <[email protected]>
Signed-off-by: Sean Paul <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]