Add missing include

source/RobotAPI/libraries/skills_gui/gui_utils.cpp

 #include "gui_utils.h"
 
-#include <mutex>
-
 #include <QLayout>
 #include <QLayoutItem>
 #include <QSplitter>

source/RobotAPI/libraries/skills_gui/gui_utils.h

@@ -9,6 +9,8 @@
 #include <QTreeWidgetItem>
 #include <QWidget>
 
+#include <mutex>
+
 #include <ArmarXCore/core/exceptions/local/ExpressionException.h>
 
 namespace armarx::gui