meson: Define TUNNEL_SINK for module-tunnel-sink

This was missing, so it was building another tunnel-source instead.

Fixes https://bugs.archlinux.org/task/63755
2 jobs for master in 5 minutes and 9 seconds (queued for 34 seconds)
Status Job ID Name Coverage
  Build
passed #636742
build-autotools

00:05:09

passed #636743
build-meson

00:02:39