cmake-gui: Fix first-configure dialog appearance in Dark Mode on Windows

Since commit 1fef953655 (Utilities/Release: Update to Qt 6.10.1 on
Windows builds, 2025-12-08) our official binaries on Windows now honor
dark mode.  However, this dialog became unreadable, seemingly due to
QTBUG-123853.  Setting the wizard style to itself seems to work around
the problem.

Fixes: #27587
Qt-Issue: https://qt-project.atlassian.net/browse/QTBUG-123853
Suggested-by: Matthew Woehlke <matthew.woehlke@kitware.com>