doc/dma-buf: Fix up include directives

Would be great if everony could add

$ make DOCBOOKS="" htmldocs

to their build scripts to catch these. 0day should also report them,
not sure why it failed to spot this.

Fixes: f54d1867 ("dma-buf: Rename struct fence to dma_fence")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Christian König <christian.koenig@amd.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161114115825.22050-4-daniel.vetter@ffwll.ch