drm: Add stackdepot include for DRM_DEBUG_MM

0day found that stackdepot.h doesn't get automatically included on all
architectures, so remember to add our #include.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Fixes: 5705670d0463 ("drm: Track drm_mm allocators and show leaks on shutdown")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Christian König <christian.koenig@amd.com>.
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161108115601.22873-1-chris@chris-wilson.co.uk

drivers/gpu/drm/drm_mm.c

@@ -105,6 +105,8 @@ static struct drm_mm_node *drm_mm_search_free_in_range_generic(const struct drm_
 						enum drm_mm_search_flags flags);
 
 #ifdef CONFIG_DRM_DEBUG_MM
+#include <linux/stackdepot.h>
+
 #define STACKDEPTH 32
 #define BUFSZ 4096