ECMGeneratePkgConfigFile API dox: fix "Since" for DESCRIPTION arg
Summary: Also place "Since" next to both argument and argument description.
Reviewers: cgiboudeaux
Reviewed By: cgiboudeaux
Subscribers: kde-frameworks-devel, kde-buildsystem
Tags: Frameworks, Build System
Differential Revision: https://phabricator.kde.org/D18913