folly/fibers/TimedMutex-inl.h: avoid shadowing warnings

Summary:
Fix a warning exposed by gcc's -Wshadow-compatible-local.

Rename inner "lockStolen" to "stolen".

Differential Revision: D4346385

fbshipit-source-id: 323cb6c96ecfe18b5540525f4124dbd75faf0b1b

folly/fibers/TimedMutex-inl.h

@@ -61,9 +61,9 @@ TimedMutex::LockResult TimedMutex::lockHelper(WaitFunc&& waitFunc) {
     auto lockStolen = [&] {
       std::lock_guard<folly::SpinLock> lg(lock_);
 
-      auto lockStolen = notifiedFiber_ != &waiter;
+      auto stolen = notifiedFiber_ != &waiter;
       notifiedFiber_ = nullptr;
-      return lockStolen;
+      return stolen;
     }();
 
     if (lockStolen) {