Subject: argon2 doesn't install pkg-config file

From: Amirouche Boubekki <[email-masked]>
Date: Thu, 2 Aug 2018 20:39:32 +0200

$ guix package -i argon2

Should install .pc file to make it possible to discover  the version of
argon2 on the host.

https://github.com/P-H-C/phc-winner-argon2/blob/master/libargon2.pc