flatpak: Fix the grilo-plugins build
The grilo-plugins build has started to fallback to downloading grilo as a subproject if certain dependencies aren't met [1]. In this case, the grilo-pls-0.3 dependency is missing because it's disabled in the grilo build via the -Denable-grl-pls=false option.
[1] grilo-plugins commit 34d3c5844864f0d0 grilo-plugins!68 (merged)