Bump Controls 2.14 in DeviceCombobox.qml

indexForValue was introduced with Qt 5.14

src/kcm/package/contents/ui/DeviceComboBox.qml

@@ -20,8 +20,8 @@
     along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
-import QtQuick 2.0
-import QtQuick.Controls 2.0
+import QtQuick 2.14
+import QtQuick.Controls 2.14
 
 ComboBox {
     textRole: "Description"