Stop using GTK3-deprecated gdk_get_display().

The new way is gdk_display_get_name(gdk_display_get_default()), which
returns a const char * rather than a char *, so I've also had to
fiddle with the prototype and call sites of get_x_display().

(Also included gtkcompat.h into uxputty.c, since that wanted to call
gdk_get_display() but didn't previously include it.)

unix/gtkwin.c

@@ -2998,7 +2998,7 @@ void cmdline_error(const char *p, ...)
     exit(1);
 }
 
-char *get_x_display(void *frontend)
+const char *get_x_display(void *frontend)
 {
     return gdk_get_display();
 }