Make out-of-line mutex contention dependent on ART_USE_FUTEXES.

Fix Mac build. Also fix Linux compilation if ART_USE_FUTEXES is disabled.

Change-Id: I51cb1d70b5548ea6121ff7567b9546bad0894e01
diff --git a/runtime/base/mutex.h b/runtime/base/mutex.h
index 25fdd59..628231a 100644
--- a/runtime/base/mutex.h
+++ b/runtime/base/mutex.h
@@ -360,10 +360,10 @@
   virtual void Dump(std::ostream& os) const;
 
  private:
+#if ART_USE_FUTEXES
   // Out-of-inline path for handling contention for a SharedLock.
   void HandleSharedLockContention(Thread* self, int32_t cur_state);
 
-#if ART_USE_FUTEXES
   // -1 implies held exclusive, +ve shared held by state_ many owners.
   AtomicInteger state_;
   // Exclusive owner. Modification guarded by this mutex.