gdbus-example-unix-fd-client: avoid strftime %c specifier

gcc 6 warns (fatally, by default) that %c only uses a 2-digit year
in some locales. The precise format does not seem to be important
for this sample code, so use ISO 8601 instead of suppressing the
warning with a pragma.

Signed-off-by: Simon McVittie <smcv@debian.org>
Reviewed-by: Colin Walters
Bug: https://bugzilla.gnome.org/show_bug.cgi?id=768453
(cherry picked from commit 037719c27c)

gio/tests/gdbus-example-unix-fd-client.c

@@ -82,7 +82,7 @@ on_name_appeared (GDBusConnection *connection,
       now = time (NULL);
       strftime (now_buf,
                 sizeof now_buf,
-                "%c",
+                "%Y-%m-%d %H:%M:%S",
                 localtime (&now));
 
       str = g_strdup_printf ("On %s, gdbus-example-unix-fd-client with pid %d was here!\n",