uapi: add missing install of dma-buf.h

As part of c11e391 "dma-buf: Add ioctls to
allow userspace to flush" a new uapi header file dma-buf.h was added, but an
entry was not added on Kbuild to install it. This patch resolves this omission
so that "make headers_install" installs this header.

Signed-off-by: Denys Dmytriyenko <denys@ti.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: David Herrmann <dh.herrmann@gmail.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Tiago Vignatti <tiago.vignatti@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1487102447-59265-1-git-send-email-denis@denix.org

include/uapi/linux/Kbuild

@@ -109,6 +109,7 @@ header-y += dlm_netlink.h
 header-y += dlm_plock.h
 header-y += dm-ioctl.h
 header-y += dm-log-userspace.h
+header-y += dma-buf.h
 header-y += dn.h
 header-y += dqblk_xfs.h
 header-y += edd.h