Fix a one-time memory leak.

This was a single object in main outside any loops, so there was
no impact on performance or anything, but obviously we still want
to patch this.

Of course, valgrind gets the credit for seeing this.

notmuch-index-message.cc

@@ -817,6 +817,8 @@ main (int argc, char **argv)
 	    }
 	}
 
+	g_io_channel_unref (channel);
+
 	gettimeofday (&tv_now, NULL);
 	elapsed = (tv_now.tv_sec - tv_start.tv_sec +
 		   (tv_now.tv_usec - tv_start.tv_usec) / 1e6);