synchronized_wrapper: Add missing return in SynchronizedRef move assignment operator

src/common/synchronized_wrapper.h

@@ -55,6 +55,7 @@ public:
     SynchronizedRef& operator=(SynchronizedRef&) = delete;
     SynchronizedRef& operator=(SynchronizedRef&& o) {
         std::swap(wrapper, o.wrapper);
+        return *this;
     }
 
     T& operator*() { return wrapper->data; }