Include panel macros to fix an error I cannot reproduce (bug #6680).

Hopefully this will fix it, but I'm not sure this was even a problem.

plugins/thunar-tpa/thunar-tpa.c

@@ -22,10 +22,17 @@
 #include <config.h>
 #endif
 
+#include <glib.h>
+
 #include <gtk/gtk.h>
+
 #include <libxfce4util/libxfce4util.h>
+
 #include <libxfce4ui/libxfce4ui.h>
+
 #include <libxfce4panel/libxfce4panel.h>
+#include <libxfce4panel/xfce-panel-macros.h>
+
 #include <thunar-tpa/thunar-tpa-bindings.h>