ecm_add_wayland_client_protocol: Improve error messages

Summary: Instead of passing the data from stdin, pass the path, this way if there's an error wayland-scanner will be able to tell us the file and line where it's failing.

Test Plan: Built kwayland and kwin with it

Reviewers: KWin, Frameworks, zzag

Reviewed By: KWin, zzag

Subscribers: kde-frameworks-devel, kde-buildsystem

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D20105