FindGperf: in ecm_gperf_generate set SKIP_AUTOMOC for generated file

Summary:
Avoids to have manually set the property on the generated source file to
deal with changed CMake policy CMP0071.

Reviewers: Frameworks, pino, apol

Reviewed By: apol

Subscribers: kde-frameworks-devel, kde-buildsystem

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D18088