kinfocenter: fix symlink to systemsettings5
kinfocenter: fix symlink to systemsettings5

As `kinfocenter`'s implementation consists of only KCMs, which are
actually just displayed using `systemsettings5`' KCM shell, the
`kinfocenter` binary is just a symlink to `systemsettings5`.

In traditional FHS environments, having a relative symlink to the
`systemsettings5` binary within the same `bin/` directory works just
fine, whereas this is broken on NixOS where `systemsettings5` resides in
a completely different `bin/` directory of the corresponding store item.

Usually CMake's `find_package` should be able to locate the
`systemsettings5` binary, but fails for reasons unknown to do so on
NixOS - so for now fixing the symlink manually as part of the `preFixup`
phase will do the job of making `kinfocenter` work again.

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