Fix compile error with Qt 5.7.1 on Debian stretch

A missing #include <QMap> cause this error:

In file included from
/home/xxx/work/umbrello/umbrello/uml1model/entityconstraint.cpp:16:0:

/home/xxx/work/umbrello/umbrello/umldoc.h:353:17: error: field

'm_diagramsToLoad' has incomplete type 'UMLDoc::DiagramsMap {aka
QMap<UMLFolder*, QList<QDomNode> >}'

DiagramsMap m_diagramsToLoad;
            ^~~~~~~~~~~~~~~~

Signed-off-by: Jean-Christian de Rivaz <jcamdr70@gmail.com>