cmake-gui: Fix crash when built with Qt 5.14 or later

In commit d7679f6427 (QCMakeCacheView: use non-deprecated List and Set
constructions, 2020-06-10, v3.18.0-rc2~13^2) the conversion of the
`this->properties()` value to QSet is incorrect for Qt 5.14+.  The
problem is that `this->properties()` returns by value, so the range
`this->properties().begin(), this->properties().end()` provides
iterators to two different instances.  Use an intermediate temporary
copy of the value to get a consistent iterator range.

Fixes: #20981
1 file changed