[KCM]Align volume controls row

Summary:
Align mute button volume slider.

Slider has 2 pixel offset, caused from wrong slider height.

Test Plan:
Before:

After:

Reviewers: Plasma, VDG, drosca, ngraham

Reviewed By: VDG, ngraham

Subscribers: broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D27645