ModelEditor: Fix build without precompiled headers

packageviewcontroller.cpp: In member function 'bool
    ModelEditor::Internal::PackageViewController::haveMatchingStereotypes(const
    qmt::MObject*, const qmt::MObject*)':
packageviewcontroller.cpp:152:43: error: invalid use of incomplete type
    'class QSet<QString>'

Change-Id: I0e61ddd6ee86e023ecfd024cf9a15178c21ceab3
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/modeleditor/packageviewcontroller.cpp

@@ -31,6 +31,8 @@
 #include "qmt/model/mdependency.h"
 #include "qmt/model_controller/modelcontroller.h"
 
+#include <QSet>
+
 namespace ModelEditor {
 namespace Internal {