[KCM]Align volume controls row

Summary:
Align mute button volume slider.

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

Test Plan:
Before:
 {F8130865, layout=inline} {F8130846, layout=inline} {F8130847, layout=inline}

After:
{F8134931, layout=inline} {F8134932, layout=inline} {F8134933, layout=inline} {F8134934, layout=inline}

Reviewers: #plasma, #vdg, drosca, ngraham

Reviewed By: #vdg, ngraham

Subscribers: broulik, plasma-devel

Tags: #plasma

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