xfce4-diskperf-plugin: update to 2.6.3

2.6.3 (2020/12/21):
------------------
- Fix build with panel 4.15+
- Fix missing-prototypes
- Remove GSourceFunc casts
- Add basic GitLab pipeline
- Update URLs from goodies.x.o to docs.x.o (Bug #16160)
- Update .gitignore
- Make build less verbose
- Translation Updates:
  Albanian, Arabic, Armenian (Armenia), Belarusian, Chinese (Taiwan),
  Croatian, Eastern Armenian, English (Canada), English (United
  Kingdom), Galician, Greek, Hebrew, Indonesian, Korean, Latvian,
  Occitan (post 1500), Panjabi (Punjabi), Polish, Portuguese (Brazil),
  Slovak, Slovenian, Spanish, Swedish, Turkish, Urdu, Urdu (Pakistan),
  Uyghur

(gutteridge)