drm/ttm: declare 'struct device' in ttm_page_alloc.h

Declare 'struct device' explicitly in ttm_page_alloc.h as this file
does not include any file declaring it. This removes the following
warning:

	warning: 'struct device' declared inside parameter list
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>

include/drm/ttm/ttm_page_alloc.h

@@ -29,6 +29,8 @@
 #include <drm/ttm/ttm_bo_driver.h>
 #include <drm/ttm/ttm_memory.h>
 
+struct device;
+
 /**
  * Initialize pool allocator.
  */