folly/fibers/test/FibersTest.cpp: avoid shadowing warnings

Summary: Fix warnings exposed by the upstream-proposed -Wshadow-compatible-local option.

Differential Revision: D4041756

fbshipit-source-id: 21faf6ad6bf39b0df42e6ef27481d1c81d37ef99

folly/fibers/test/FibersTest.cpp

@@ -1564,7 +1564,7 @@ TEST(FiberManager, semaphore) {
 
       for (size_t i = 0; i < kTasks; ++i) {
         manager.addTask([&, completionCounter]() {
-          for (size_t i = 0; i < kIterations; ++i) {
+          for (size_t j = 0; j < kIterations; ++j) {
             sem.wait();
             ++counter;
             sem.signal();