(font_unparse_gtkname): Add prototype.

src/font.h

@@ -822,6 +822,7 @@ extern int font_unparse_xlfd P_ ((Lisp_Object font, int pixel_size,
 extern int font_parse_fcname P_ ((char *name, Lisp_Object font));
 extern int font_unparse_fcname P_ ((Lisp_Object font, int pixel_size,
 				  char *name, int bytes));
+extern int font_unparse_gtkname P_ ((Lisp_Object, struct frame *, char *, int));
 extern void register_font_driver P_ ((struct font_driver *driver, FRAME_PTR f));
 extern void free_font_driver_list P_ ((FRAME_PTR f));
 extern Lisp_Object font_update_drivers P_ ((FRAME_PTR f, Lisp_Object list));