isostream: don't forget to fflush.

The only difference this makes is that status updates show up in the log
more frequently when redirected through logos.

Change-Id: I363d364dafe7fda18135db2990b5cdb557808b84
diff --git a/cmds/isostream.c b/cmds/isostream.c
index 9ffbf19..795646f 100644
--- a/cmds/isostream.c
+++ b/cmds/isostream.c
@@ -309,6 +309,7 @@
                stats.drop_count,
                stats.drop_maxlength / 1e6,
                stats.drop_maxdepth / 1e6);
+        fflush(stdout);
         last_print_time = now;
       }
     }