folly/experimental/test/BitsBenchmark.cpp: avoid -Wsign-compare error

folly/experimental/test/BitsBenchmark.cpp: avoid -Wsign-compare error

Summary:
* folly/experimental/test/BitsBenchmark.cpp (benchmarkSet):
Another s/int/size_t/ change to type of for-loop index, to match
the type of the loop's upper bound.

Test Plan:
Run this and note there are fewer errors than before:
fbconfig --platform-all=gcc-4.9-glibc-2.20 -r folly && fbmake dbgo

Reviewed By: davejwatson@fb.com

Subscribers: folly-diffs@

FB internal diff: D1767853

Tasks: 5941250

Signature: t1:1767853:1420587945:f32a0ef33f6c5fa75598ecb114a243f83b31e550