Fix GCC error: control reaches end of non-void function [-Werror=return-type]

--- src/attributeitem.cpp
+++ src/attributeitem.cpp
@@ -87,6 +87,7 @@ QString AttributeItem::getAttributeTypeString()
         case Undefined:
             return "Undefined";
         }
+    return "Undefined";
     }
 
     void AttributeItem::setInternal(bool b) { internal = b; }