event/ServerSocket: use C++11 initializer

src/event/ServerSocket.cxx

@@ -198,7 +198,7 @@ OneServerSocket::Open()
 }
 
 ServerSocket::ServerSocket(EventLoop &_loop) noexcept
-	:loop(_loop), next_serial(1) {}
+	:loop(_loop) {}
 
 /* this is just here to allow the OneServerSocket forward
    declaration */

src/event/ServerSocket.hxx

@@ -39,7 +39,7 @@ class ServerSocket {
 
 	std::list<OneServerSocket> sockets;
 
-	unsigned next_serial;
+	unsigned next_serial = 1;
 
 public:
 	ServerSocket(EventLoop &_loop) noexcept;