drm/qxl: Fix randbuild error

If DEM_QXL is y and DRM_TTM_HELPER is m, building fails:

drivers/gpu/drm/qxl/qxl_object.o: undefined reference to `drm_gem_ttm_print_info'

Select DRM_TTM_HELPER to fix this.

Fixes: 78d54f1f6a33 ("drm/qxl: use drm_gem_ttm_print_info")
Signed-off-by: YueHaibing <[e-mail]>
Link: http://patchwork.freedesktop.org/patch/msgid/[e-mail]
Signed-off-by: Gerd Hoffmann <[e-mail]>

drivers/gpu/drm/qxl/Kconfig

···
 	depends on DRM && PCI && MMU
 	select DRM_KMS_HELPER
 	select DRM_TTM
+	select DRM_TTM_HELPER
 	select CRC32
 	help
 	  QXL virtual GPU for Spice virtualization desktop integration.