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
diff --git a/src/modules/meson.build b/src/modules/meson.build
index 92d5871..dd2241f 100644
--- a/src/modules/meson.build
+++ b/src/modules/meson.build
@@ -55,7 +55,7 @@
   [ 'module-suspend-on-idle', 'module-suspend-on-idle.c' ],
   [ 'module-switch-on-connect', 'module-switch-on-connect.c' ],
   [ 'module-switch-on-port-available', 'module-switch-on-port-available.c' ],
-  [ 'module-tunnel-sink', 'module-tunnel.c', [], [], [x11_dep] ],
+  [ 'module-tunnel-sink', 'module-tunnel.c', [], ['-DTUNNEL_SINK=1'], [x11_dep] ],
   [ 'module-tunnel-sink-new', 'module-tunnel-sink-new.c' ],
   [ 'module-tunnel-source', 'module-tunnel.c', [], [], [x11_dep] ],
   [ 'module-tunnel-source-new', 'module-tunnel-source-new.c' ],