Fix build using gcc 10 -fno-common flag

src/terminal-app.c

@@ -68,6 +68,8 @@
  *
  */
 
+GSettings *settings_global;
+
 struct _TerminalAppClass
 {
 	GObjectClass parent_class;

src/terminal-app.h

@@ -83,7 +83,7 @@ G_BEGIN_DECLS
 typedef struct _TerminalAppClass TerminalAppClass;
 typedef struct _TerminalApp TerminalApp;
 
-GSettings *settings_global;
+extern GSettings *settings_global;
 
 GType terminal_app_get_type (void);