aomenc: flush stderr after printing

BUG=aomedia:2909

Signed-off-by: Christopher Degawa <christopher.degawa@intel.com>
Change-Id: Ibde4098a3a22517caf428ae37b30f5312ac126ed
diff --git a/apps/aomenc.c b/apps/aomenc.c
index 02732ec..14f6ace 100644
--- a/apps/aomenc.c
+++ b/apps/aomenc.c
@@ -2257,6 +2257,7 @@
                   cx_time > 9999999 ? "ms" : "us", fps >= 1.0 ? fps : fps * 60,
                   fps >= 1.0 ? "fps" : "fpm");
           print_time("ETA", estimated_time_left);
+          fflush(stderr);
         }
 
       } else {