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>