LU-1606 api: Include stddef.h in ll_fiemap.h

LU-1606 api: Include stddef.h in ll_fiemap.h

In user space only, include stddef.h in ll_fiemap.h to define
such things as size_t.

Signed-off-by: Christopher J. Morrone <morrone2@llnl.gov>
Change-Id: Ia5efa7758d27952009a50c479b3e02d4b6b7c9b6
Reviewed-on: http://review.whamcloud.com/3425
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Li Wei <liwei@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>