Documentation: include sync_file into DocBook

Add entry in device-drivers.tmpl for sync_file documentation.
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/DocBook/device-drivers.tmpl

@@ -136,6 +136,8 @@ X!Edrivers/base/interface.c
 !Iinclude/linux/seqno-fence.h
 !Edrivers/dma-buf/reservation.c
 !Iinclude/linux/reservation.h
+!Edrivers/dma-buf/sync_file.c
+!Iinclude/linux/sync_file.h
 !Edrivers/base/dma-coherent.c
 !Edrivers/base/dma-mapping.c
      </sect1>