event/ServerSocket: pass UniqueSocketDescriptor by value

Passing it by value is actually smaller (32 bit) than the rvalue
reference (64 bit pointer), and it ensures that the object is consumed
after the call returns, no matter how the methods are implemented.

src/event/ServerSocket.hxx

@@ -117,7 +117,7 @@ public:
 	void Close();
 
 protected:
-	virtual void OnAccept(UniqueSocketDescriptor &&fd,
+	virtual void OnAccept(UniqueSocketDescriptor fd,
 			      SocketAddress address, int uid) = 0;
 };