Include class keyword for friend declaration

Only C++11 allows omitting the class keyword, so include it so
the code compiles with older compilers.

common/os/Mutex.h

@@ -31,7 +31,7 @@ namespace os {
     void unlock();
 
   private:
-    friend Condition;
+    friend class Condition;
 
     void* systemMutex;
   };