[ostree] Fix build failure on g_autoptr(gchar) with glib master

[Alexander Larsson <alexl src gnome org>]

commit d26359f823b6aeda2cf9d391cb117a0a509b91d3
Author: Alexander Larsson <alexl redhat com>
Date:   Fri Mar 20 15:36:56 2015 +0100

    Fix build failure on g_autoptr(gchar) with glib master
    
    This was removed in favour of g_autofree

 src/ostree/ot-builtin-show.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/src/ostree/ot-builtin-show.c b/src/ostree/ot-builtin-show.c
index 4eb52f3..d45bd67 100644
--- a/src/ostree/ot-builtin-show.c
+++ b/src/ostree/ot-builtin-show.c
@@ -155,7 +155,7 @@ print_signature (OstreeGpgVerifyResult *result,
   g_autoptr(GVariant) variant = NULL;
   g_autoptr(GDateTime) date_time_utc = NULL;
   g_autoptr(GDateTime) date_time_local = NULL;
-  g_autoptr(gchar) formatted_date_time = NULL;
+  g_autofree char *formatted_date_time = NULL;
   gint64 timestamp;
   gint64 exp_timestamp;
   const char *fingerprint;