Fix warning in IPC proxy classes

src/lib/ipc/IpcClientProxy.cpp

@@ -146,7 +146,7 @@ IpcClientProxy::send(const IpcMessage& message)
 	switch (message.type()) {
 	case kIpcLogLine: {
 		const IpcLogLineMessage& llm = static_cast<const IpcLogLineMessage&>(message);
-		String logLine = llm.logLine();
+		const String logLine = llm.logLine();
 		ProtocolUtil::writef(&m_stream, kIpcMsgLogLine, &logLine);
 		break;
 	}

src/lib/ipc/IpcServerProxy.cpp

@@ -94,7 +94,7 @@ IpcServerProxy::send(const IpcMessage& message)
 
 	case kIpcCommand: {
 		const IpcCommandMessage& cm = static_cast<const IpcCommandMessage&>(message);
-		String command = cm.command();
+		const String command = cm.command();
 		ProtocolUtil::writef(&m_stream, kIpcMsgCommand, &command);
 		break;
 	}