[3.0] contrib: qt: Backport a patch to fix compiling with latest libc++
The class std::unary_function was deprecated in C++11 and is removed in C++17. The latest libc++ has caught up with this and now hides this class when building in C++17 or C++20 mode.
Backport a patch from Qt v5.11.1 that removes usage of this class.
Edited by Felix Paul Kühne