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