drm/ttm: make ttm_caching.h self-contained

Include <linux/pgtable.h> for pgprot_t.

Cc: Christian Koenig <christian.koenig@amd.com>
Cc: Huang Rui <ray.huang@amd.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/fb87ab4b4490c53e9ece66d53c4f178ead244cb5.1709898638.git.jani.nikula@intel.com