[conan] Set qt version 5.12.5@bincrafters/stable.

This should fix an error by conan:

ERROR: E:\Conan.conan\data\glib\2.58.3\bincrafters\stable\export\conanfile.py: Error while initializing requirements. Wrong package recipe reference zlib/1.2.11 Write something like OpenCV/1.0.6@user/stable