[mutter/wayland] main: Squash constness warning

[Jasper St. Pierre <jstpierre src gnome org>]

commit 0a9754f305a356b1ffb7737bfaa3f285ceec9148
Author: Jasper St. Pierre <jstpierre mecheye net>
Date:   Mon Feb 3 18:02:16 2014 -0500

    main: Squash constness warning

 src/core/main.c |    8 ++++++--
 1 files changed, 6 insertions(+), 2 deletions(-)
---
diff --git a/src/core/main.c b/src/core/main.c
index a730152..82ae483 100644
--- a/src/core/main.c
+++ b/src/core/main.c
@@ -345,9 +345,13 @@ meta_clutter_init (void)
  * also is %NULL, use the default - :0.0
  */
 static void
-meta_select_display (gchar *display_name)
+meta_select_display (char *display_arg)
 {
-  if (!display_name)
+  const char *display_name;
+
+  if (display_arg)
+    display_name = (const char *) display_arg;
+  else
     display_name = g_getenv ("MUTTER_DISPLAY");
 
   g_setenv ("DISPLAY", display_name, TRUE);