Fix build with -fno-common

Index: src/gui.c
--- src/gui.c.orig
+++ src/gui.c
@@ -42,6 +42,8 @@
 
 extern preferencestype preferences;
 
+guitype *gui;
+
 static void on_highcheck_toggled (GtkToggleButton *togglebutton, gpointer user_data);
 static void on_soundcheck_toggled (GtkToggleButton *togglebutton, gpointer user_data);
 
