wayland-private: Make display_name non-const

The string used to point to memory owned by libwayland-server, but
with the ability to override the display name, we took over ownership
by copying the string as necessary.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/176

src/wayland/meta-wayland-private.h

@@ -64,7 +64,7 @@ typedef struct
 struct _MetaWaylandCompositor
 {
   struct wl_display *wayland_display;
-  const char *display_name;
+  char *display_name;
   GHashTable *outputs;
   struct wl_list frame_callbacks;