Add FindSeccomp to find-modules

Summary: This is copied from KScreenlocker, but will be utilized in Baloo too.

Test Plan:

• Autotests are working
• KScreenlocker and Baloo build with seccomp enabled

Reviewers: graesslin, cgiboudeaux

Reviewed By: cgiboudeaux

Subscribers: cgiboudeaux, Frameworks, Build System

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D8998