Bug#954374: Breaks on systems with /usr merge

[Fiona Klute <fiona.klute@gmx.de>]

This change appears to break systems with /usr merge [1], I get this
when trying to upgrade libc6 from 2.30-2 to 2.30-3 on amd64:

> A copy of the C library was found in an unexpected directory:
>   '/usr/lib/x86_64-linux-gnu/ld-2.30.so'
> It is not safe to upgrade the C library in this situation;
> please remove that copy of the C library or get it out of
> '/usr/lib/x86_64-linux-gnu' and try again.

It seems like the preinst script is getting confused by /lib being a
symlink to /usr/lib:

$ dpkg -L libc6 | grep ld-2.30.so
/lib/x86_64-linux-gnu/ld-2.30.so
$ realpath /lib/x86_64-linux-gnu/ld-2.30.so
/usr/lib/x86_64-linux-gnu/ld-2.30.so

[1] https://wiki.debian.org/UsrMerge