fix compilation on OSX (in the wake of eaca325c)

diff --git a/libs/ardour/route_group.cc b/libs/ardour/route_group.cc
index 5b5a8f25b184e4220c986811fb205116f2cb42a6..9ad5aa91e6cfc16fd7caa47b0ad91446707b42c7 100644 (file)
--- a/libs/ardour/route_group.cc
+++ b/libs/ardour/route_group.cc
@@ -24,6 +24,7 @@
 #include "pbd/error.h"
 #include "pbd/enumwriter.h"
 #include "pbd/strsplit.h"
+#include "pbd/debug.h"
 
 #include "ardour/amp.h"
 #include "ardour/audio_track.h"