dmabuf: Mark up onstack timer for selftests

The dma-fence selftest uses an on-stack timer that requires explicit
annotation for debugobjects.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111442


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190820122118.13698-1-chris@chris-wilson.co.uk