Added back setting of QT_QMAKE_EXECUTABLE in UseParaView.cmake

This was inadvertently removed in commit
eed7ccd3f7e804fed3864fcbe54fbd6958e63b41

Change-Id: I46ddec948e892cb507bf34549ae62b1b530e345e
Merge topic '15043_add_qmake_variable_back' into next

2a85f0c4 Added back setting of QT_QMAKE_EXECUTABLE in UseParaView.cmake