Drop Qt module name from include statements.

This is a preparation to be able to build KeePassX against Qt 5.

src/core/TimeDelta.cpp

@@ -17,7 +17,7 @@
 
 #include "TimeDelta.h"
 
-#include <QtCore/QDateTime>
+#include <QDateTime>
 
 QDateTime operator+(const QDateTime& dateTime, const TimeDelta& delta) {
     return dateTime.addDays(delta.getDays())