Marked reserved GObject class slots as private

This stops GTK-Doc from complaining

src/gswe-moment.h

@@ -145,6 +145,7 @@ struct _GsweMomentClass {
     /* Parent class structure */
     GObjectClass parent_class;
 
+    /*< private >*/
     /* Signals */
     void (*changed)(GsweMoment *moment);
 

src/gswe-timestamp.h

@@ -49,6 +49,7 @@ struct _GsweTimestampClass {
     /* Parent class */
     GObjectClass parent_class;
 
+    /*< private >*/
     /* Signals */
     void (*changed)(GsweTimestamp *self);