stat: fix unified_rw_reporting=both BW and IOPS

stat: fix unified_rw_reporting=both BW and IOPS

When unified_rw_reporting=both, the value for the significant_figures
option is not populated in the struct thread_stat containing the mixed
r/w/t data. Thus, when bandwidth and IOPS numerical values are converted
to strings, they are converted to "0".

This patch populates the significant figures member of struct
thread_stat for the mixed r/w/t data.

Fixes: https://github.com/axboe/fio/issues/1844
Signed-off-by: Vincent Fu <vincent.fu@samsung.com>