drm/ttm: fix warning in new sys man

Include the header for the prototype.
Signed-off-by: Christian König <christian.koenig@amd.com>
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210503142710.153369-1-christian.koenig@amd.com

drivers/gpu/drm/ttm/ttm_sys_manager.c

@@ -4,6 +4,8 @@
 #include <drm/ttm/ttm_device.h>
 #include <drm/ttm/ttm_placement.h>
 
+#include "ttm_module.h"
+
 static int ttm_sys_man_alloc(struct ttm_resource_manager *man,
 			     struct ttm_buffer_object *bo,
 			     const struct ttm_place *place,