fix: flush spdlog messages also when logging to stderr (KDAB#142)

The existing code was only enabling flush when logs are redirected to files.
Without this, the relative order of the debug output from qWarnings() and from Message.log() wasn't preserved, making debugging difficult.

src/core/knutcore.cpp

@@ -193,6 +193,8 @@ void KnutCore::initialize(Settings::Mode mode)
     new ScriptManager(this);
     if (Core::Settings::instance()->value<bool>(Core::Settings::SaveLogsToFile))
         initializeMultiSinkLogger();
+    // auto flush when "info" or higher message is logged.
+    spdlog::flush_on(spdlog::level::info);
     m_initialized = true;
 }
 
@@ -207,8 +209,6 @@ void KnutCore::initializeMultiSinkLogger()
         Core::Settings::instance()->logFilePath().toStdString(), SIZE_MAX, max_files, rotate_on_open);
     auto logger = spdlog::default_logger();
     logger->sinks().push_back(fileSink);
-    // auto flush when "info" or higher message is logged.
-    spdlog::flush_on(spdlog::level::info);
 }
 
 } // namespace Core