Can't build Messaging module under M5 if QMF is available

For Maemo 5 / Fremantle, if the QMF is available (i.e. the libqmf-dev package is installed), we can't build the Messaging module for Qt Mobility. The reason is that the QMessagePrivate (defined in the qmessage_p.h file) is different if QMF is enabled, but the implementation file for M5, qmessage_maemo.cpp doesn't care whether QMF is enabled or not.