shut up warning (class/struct mismatch)

Index: src/calf/giface.h
--- src/calf/giface.h.orig
+++ src/calf/giface.h
@@ -282,8 +282,9 @@ struct plugin_metadata_iface
 };
 
 /// Interface for host-GUI-plugin interaction (should be really split in two, but ... meh)
-struct plugin_ctl_iface: public virtual plugin_metadata_iface
+class plugin_ctl_iface: public virtual plugin_metadata_iface
 {
+public:
     /// @return value of given parameter
     virtual float get_param_value(int param_no) = 0;
     /// Set value of given parameter
