ECMAddAppIcon: Add sc in regex to extract extension from valid names

Summary: Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

Reviewers: tcanabrava, apol

Reviewed By: tcanabrava, apol

Subscribers: kde-frameworks-devel, kde-buildsystem

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D26751