FindQt4: Fix QT_QMAKE{_QMAKE => }_EXECUTABLE typo

The parent commit accidentally referenced QT_QMAKE_QMAKE_EXECUTABLE
which does not exist.  Use QT_QMAKE_EXECUTABLE instead.

Reported-by: m.hergarden@euphoria-it.nl
Merge topic 'findqt4-withqt5' into next

161a793 FindQt4: Fix QT_QMAKE{_QMAKE => }_EXECUTABLE typo