[sddm-theme] Fix fontsize of virtual keyboard ToolButton

Summary: When we updated the fonts in the sddm-theme, we bumped the default size from 9pt to 10pt. We missed adding the relevant code to the virtual keyboard button so it remained 9pt. This patch brings it up to 10pt.

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21614