sbin install path should be relocatable (cmake)

[foxlet]

Currently ntfs2btrfs gets installed to sbin by default, however some distributions (e.g. Void Linux) do not allow this as they symlink sbin to bin. A cmake flag to set this would be useful in order to get it packaged.

[maharmstone]

I'm guessing from void-linux/void-packages#33396 that it's confusing their build system - CMake of course has no problems with symlinks itself.

Does f972474 sort this?

[foxlet]

Yep, that works, thanks :)