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/uxputty.c

@@ -7,11 +7,14 @@
 #include <stdlib.h>
 #include <assert.h>
 #include <unistd.h>
+#include <gtk/gtk.h>
 #include <gdk/gdk.h>
 
 #include "putty.h"
 #include "storage.h"
 
+#include "gtkcompat.h"
+
 /*
  * Stubs to avoid uxpty.c needing to be linked in.
  */