meson: Change default install mode for roc

The roc-toolkit is an exotic project. Most distrobutions do not include it in their archives. Even Arch-Linux only has a single broken AUR package for roc. As stated in commit d7cddbdb, many distributions set --auto_features=enabled, and that combined with a difficult dependency to satisfy with roc, will cause pipewire to fail to build on most systems. Using the same logic as the above commit, building the pulseaudio roc-modules should be disabled by default and only enabled by the few that require them.