libkate: Allow building without python symlink
libkate: Allow building without python symlink

This tells libkate to use python3 rather then default python. This allow
building without python symlink.

nirbheek/cerbero,atsushieno/cerbero,fluendo/cerbero,fluendo/cerbero,atsushieno/cerbero,GStreamer/cerbero,atsushieno/cerbero,atsushieno/cerbero,GStreamer/cerbero,fluendo/cerbero,fluendo/cerbero,nirbheek/cerbero,GStreamer/cerbero,GStreamer/cerbero,nirbheek/cerbero,atsushieno/cerbero,GStreamer/cerbero,nirbheek/cerbero,fluendo/cerbero