Since 'QObject' and the 'Q_OBJECT' macro are used,
the corresponding header needs to be included.

Without the include and forward-declaration given here,
the compiler doesn't know about 'QObject' and 'QAction'
which are used further below, unless another header that
takes care of this is included before 'QtMenu.hxx'.

The error could be provoked e.g. by moving the
'#include <Qt5Menu.hxx>' in front of all other includes
in 'vcl/qt5/Qt5Frame.cxx'.

Change-Id: I76c15eb445cbe26774d6e4177e12c93658daf0ad
Reviewed-on: https://gerrit.libreoffice.org/65130
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>