cmake-gui: Fix theme on Windows with Qt >= 5.10

The QWindowsVistaStyle moved into a separate plugin in Qt 5.10 (see https://code.qt.io/cgit/qt/qtbase.git/commit/?id=4f3249f32dbe5c20aabbfd9b4f9c558aaf449e48).

We need to link explicitly against this plugin now after updating to Qt 5.12 (!3079 (merged)).

Note: There might be other missing libraries to link against in win32_release.cmake and win64_release.cmake, but I can't test this, since the specific Qt configurations for the release builds are not public (#19103 (closed)).

Fixes: #19147 (closed)
Topic-rename: cmake-gui-windows-theme