netperf: set _FILE_OFFSET_BITS to 64 to support large files

Without this patch, netperf2.6 can only
support files smaller than 2GB with TCP_SENDFILE item.

Signed-off-by: Ting Liu <b28495@freescale.com>
Tested-by: Jianhua <Xie&lt;jianhua.xie@freescale.com>
Reviewed-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Joe MacDonald <joe@deserted.net>