qapi: add release designator to gluster logfile option

The "logfile" option to BlockdevOptionsGluster will not be in
QEMU until 2.8.  Update comment to indicate this.
Reported-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NJeff Cody <jcody@redhat.com>

qapi/block-core.json

@@ -2197,7 +2197,7 @@
 #
 # @debug-level: #optional libgfapi log level (default '4' which is Error)
 #
-# @logfile:     #optional libgfapi log file (default /dev/stderr)
+# @logfile:     #optional libgfapi log file (default /dev/stderr) (Since 2.8)
 #
 # Since: 2.7
 ##