drm/ttm: include linux/seq_file.h for seq_printf

Fixes

drivers/gpu/drm/ttm/ttm_page_alloc.c: In function 'ttm_page_alloc_debugfs':
drivers/gpu/drm/ttm/ttm_page_alloc.c:829: error: implicit declaration of
function 'seq_printf'

Signed-off-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>