Copy files rather than broken symlinks

This fixes installation on Windows. An alternative solution would have been to dynamically create the symlinks in the install step, on Linux, and to copy them on Windows.

01:05:07.444 CMake Error at src/desktoptheme/breeze-dark/cmake_install.cmake:43 (file): 01:05:07.444 file INSTALL cannot read symlink 01:05:07.444 "C:/_/acd4f350/plasma-framework-5.71.0/src/desktoptheme/breeze-dark/widgets/plasmoidheading.svgz" 01:05:07.444 to duplicate at 01:05:07.444 "C:/Craft/BC/windows-mingw_64-gcc/build/kde/frameworks/tier3/plasma-framework/image-RelWithDebInfo-5.71.0/Craft/BC/windows-mingw_64-gcc/bin/data/plasma/desktoptheme/breeze-dark/widgets/plasmoidheading.svgz": 01:05:07.444 No such file or directory. 01:05:07.444 Call Stack (most recent call first): 01:05:07.444 src/desktoptheme/cmake_install.cmake:40 (include) 01:05:07.444 src/cmake_install.cmake:41 (include) 01:05:07.444 cmake_install.cmake:109 (include)