fixed return type in gui plugin manager to make compiler happy

Author: Simon Klix

plugins/gui/include/gui/plugin_relay/gui_plugin_manager.h

@@ -156,7 +156,7 @@ namespace hal {
         void loadFeature(FacExtensionInterface::Feature ft, const QString& extension=QString());
         SupportedFileFormats listFacFeature(FacExtensionInterface::Feature ft) const;
         GuiPluginEntry* at(int irow) const;
-        int addExternalPlugin(const QString& path);
+        void addExternalPlugin(const QString& path);
         void removeEntry(int irow);
    };