drm/ttm: make ttm_caching.h self-contained

Include <linux/pgtable.h> for pgprot_t.

Cc: Christian Koenig <[e-mail]>
Cc: Huang Rui <[e-mail]>
Acked-by: Thomas Zimmermann <[e-mail]>
Reviewed-by: Christian König <[e-mail]>
Reviewed-by: Alex Deucher <[e-mail]>
Signed-off-by: Jani Nikula <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/fb8[e-mail]

include/drm/ttm/ttm_caching.h

···
 #ifndef _TTM_CACHING_H_
 #define _TTM_CACHING_H_
 
+#include <linux/pgtable.h>
+
 #define TTM_NUM_CACHING_TYPES	3
 
 /**