drm/syncobj: Include the prototype for drm_timeout_abs_to_jiffies()

Sparse complains:
../drivers/gpu/drm/drm_syncobj.c:942:13: warning: symbol 'drm_timeout_abs_to_jiffies' was not declared. Should it be static?

Include the correct header with the prototype.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190710125143.9965-4-ville.syrjala@linux.intel.com
Reviewed-by: Sean Paul <sean@poorly.run>