xfce4-panel-plugin-diskperf: update to version 2.5.4
xfce4-panel-plugin-diskperf was updated to version 2.5.4, this mainly fixes https://bugzilla.xfce.org/show_bug.cgi?id=8882 (Progress bars don't show anything) and implements a unified look following the HIG
- Submitted by Guido Berhoerster (gberh)