KStyle: Fix QQC2 ProgressBar desktop style in RTL layout direction

I think it must be correct, but it's the first time I'm actually touching Breeze sources.

Before:

KStyle_ProgressBar_RTL

After (don't mind the «Before» & «After» labels on the video):

Breeze_ProgressBar_RTL

BUG: 430101

CC: @ndavis @ngraham