Fix build with Qt 5.15

Authored by apol on Feb 14 2020, 3:30 PM.

Description

Fix build with Qt 5.15

Summary: QComboBox::currentIndexChanged(int) got deprecated.

Test Plan: Builds

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D27406