Fix size of SpinBoxes

Summary:
The numbers in the text entry box for "Manual Brightness Step" in the Redshift System Tray controller don't fit properly.

BUG: 413218

Test Plan:

1. right click on Redshift System Tray icon
2. Click on "Configure Redshift Control

Observed:
Numbers in text entry box for "Manual Brightness Step" is cut off

Expected:
Numbers should be wholly visible.

Reviewers: martinkostolny, broulik, Plasma, ngraham

Reviewed By: ngraham

Subscribers: ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24861