Add find module for Google's libphonenumber

Summary:
This is currently in use by plasma-phone-components, but will also be
needed by KItinerary.

Reviewers: #build_system, #frameworks, cgiboudeaux

Reviewed By: cgiboudeaux

Subscribers: cgiboudeaux, kde-frameworks-devel, kde-buildsystem

Tags: #frameworks, #build_system

Differential Revision: https://phabricator.kde.org/D16954