Allow to build Sonnet without Qt5Widgets

Summary:
This is useful for cross-compilation, where we only need the QtCore-only
parsetrigrams build tool on the host and avoids having to pull in a full
graphics stack for the host build in Yocto.

Reviewers: Frameworks, cordlandwehr, aacid

Reviewed By: aacid

Subscribers: aacid

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D6990