Bug#892999: Provide pkg-config.multiarch

[Corentin Noël]

Package: pkgconf
Version: 0.9.12-6

The pkg-config package provides /usr/lib/pkg-config.multiarch file when
pkgconf provides /usr/lib/pkgconf.multiarch
I'm currently writing a function trying to "guess" the pkgconfig
folders in a sysroot and having to read only one file reliably would be
better.
pkgconf could provide /usr/lib/pkg-config.multiarch the same way that
it provides the /usr/bin/pkg-config substitution