folly/gen/Parallel-inl.h: trivial -Wsign-compare fix

folly/gen/Parallel-inl.h: trivial -Wsign-compare fix

Summary:
* folly/gen/Parallel-inl.h (folly): Use a for-loop index of type
size_t (not "int") to match type of upper bound.
Otherwise, gcc-4.9 fails with this:
folly/gen/Parallel-inl.h:242:27: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]

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: D1767160

Tasks: 5941250

Signature: t1:1767160:1420577886:6acfb7f6d50ed715dcf4058841aeabc39fa99cbc