The tag is present in Lintian version 2.104.244. That is the most recent version we know about.
The arch all pkg-config file contains a reference to a multi-arch path.
This can be usually be fixed by moving this file to a multi-arch path.
Another likely cause is using debhelper 9 or newer (thus enabling multi-arch paths by default) on a package without multi-arch support. The usual cure in this case is to update it for multi-arch.
Last but not least, this file could contain a reference to a cross architecture (like for instance an x86_64-linux-gnu pkg-config file referencing an i386-linux-gnu file). In this case the usual cure is to fix this path.