drm/amdgpu/display: include slab.h in dcn21_resource.c

It's apparently needed in some configurations.
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dcn21/dcn21_resource.c

@@ -23,6 +23,8 @@
  *
  */
 
+#include <linux/slab.h>
+
 #include "dm_services.h"
 #include "dc.h"