Revert "QtDialog: Don't define KWSYS_CP_UTF8 if using Qt 5."

This reverts commit 5263e765f6650b36aeeec4bcbfc102471065d728.
Merge topic 'fix-Qt5-windows-build' into next

888a35f6 Revert "QtDialog: Don't define KWSYS_CP_UTF8 if using Qt 5."