Support starting the killer_helper from build dir

Summary:
As with all libexec executables we first need to test whether the binary
is available in the application's directory and start this, otherwise
the system wide installed one.

See: https://community.kde.org/Guidelines_and_HOWTOs/Making_apps_run_uninstalled

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16763