drm/i915/guc: Fix build break on config without DEBUG_FS

In commit 56b9a8b0 ("drm/i915/guc: Update syntax of GuC
log functions") we accidentally removed debugfs.h header
where needed stub functions were defined.

Reported-by: Mike Lothian <mike@fireburn.co.uk>
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Mike Lothian <mike@fireburn.co.uk>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180315152848.40476-1-michal.wajdeczko@intel.com