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

Summary:
[just like https://phabricator.fb.com/D1767160 --
I don't know why I didn't see this the first time]
* 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: philipp@fb.com

Subscribers: folly-diffs@

FB internal diff: D1767520

Signature: t1:1767520:1420583501:121f7212f78774adb6ca7cf67cbab83bf604cbbe

folly/gen/ParallelMap-inl.h

@@ -75,7 +75,7 @@ class PMap : public Operator<PMap<Predicate>> {
         : pred_(pred),
           pipeline_(nThreads, nThreads) {
         workers_.reserve(nThreads);
-        for (int i = 0; i < nThreads; i++) {
+        for (size_t i = 0; i < nThreads; i++) {
           workers_.push_back(std::thread([this] { this->predApplier(); }));
         }
       }