Fixed outdated qt-gstreamer link

[genestealer]

Description:

Path no longer valid: anongit.freedesktop.org/gstreamer/qt-gstreamer". Corrected to official mirror.

Related issue (if applicable): fixes #

Checklist:

• [x] The code change is tested and works locally.

[robert5974]

When did the link expire?

I have run an install many times in the last week or so. It's been a few days since I have done it.

Had to be recent I assume.