Fix folly/fibers/test/FibersTest.cpp under gcc8 with -Wshadow

Summary:
[Folly] Fix `folly/fibers/test/FibersTest.cpp` under gcc8 with `-Wshadow`.

```
folly/fibers/test/FibersTest.cpp: In lambda function:
folly/fibers/test/FibersTest.cpp:1322:39: error: declaration of 'rctx' shadows a previous local [-Werror=shadow]
folly/fibers/test/FibersTest.cpp:1320:37: note: shadowed declaration is here
```

Reviewed By: kirkshoop

Differential Revision: D13648322

fbshipit-source-id: 1c27f92f471e5a758cd4e58e5ca4b3388f9e9763

folly/fibers/test/FibersTest.cpp

@@ -1319,7 +1319,7 @@ TEST(FiberManager, RequestContext) {
   {
     folly::RequestContextScopeGuard rctx;
     fm.addTask([&]() {
-      folly::RequestContextScopeGuard rctx;
+      folly::RequestContextScopeGuard rctx2;
       auto rcontext4 = folly::RequestContext::get();
       baton4.wait();
       EXPECT_EQ(rcontext4, folly::RequestContext::get());