amdkfd: explicitely include io.h in kfd_doorbell.c

This patch fixes a compilation error when using certain configuration by
including the file io.h in kfd_doorbell.c

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@amd.com>

drivers/gpu/drm/amd/amdkfd/kfd_doorbell.c

@@ -23,6 +23,7 @@
 #include <linux/mm.h>
 #include <linux/mman.h>
 #include <linux/slab.h>
+#include <linux/io.h>
 
 /*
  * This extension supports a kernel level doorbells management for