Icon name should be set as read-only on Weblate

https://translate.fedoraproject.org/translate/pulseaudio/paprefs/en/?checksum=8627763efe39e55f

The icon name is extracted from src/paprefs.desktop.in and is made available for the translators in Weblate. This presents the risk of getting translated to another thing that does not match the icon filename.

Weblate features adding read-only flat to a string so the translators can not translate it. See https://docs.weblate.org/en/latest/formats.html#read-only-strings

Please consider setting it to read-only.