uapi: add missing install of dma-buf.h

uapi: add missing install of dma-buf.h

As part of caeb743c4474e690f6728f17ff08fea180ff256d "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