Fix unnecessary soversion change

Summary:
Commit fd1e2505 only bumped the library version, but didn't break
ABI. So we don't need to bump the soversion as that commit (implicitly) did.

This patch restores the old soversion by explicitly setting it in
ecm_setup_version(), which otherwise would use `${KDE_APPLICATIONS_VERSION_MAJOR}`
as fallback.

Test Plan: Build/istall baloo-widgets.

Reviewers: arojas

Subscribers: #baloo

Tags: #baloo

Differential Revision: https://phabricator.kde.org/D14258