[pitivi] Update gst-transcoder

[Thibault Saunier <tsaunier src gnome org>]

commit c8cce436220659901cb6c0b35ee3d199586f16d2
Author: Thibault Saunier <tsaunier gnome org>
Date:   Sun Apr 30 15:03:07 2017 -0300

    Update gst-transcoder

 subprojects/gst-transcoder |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/subprojects/gst-transcoder b/subprojects/gst-transcoder
index 63adfd8..a39c3e8 160000
--- a/subprojects/gst-transcoder
+++ b/subprojects/gst-transcoder
@@ -1 +1 @@
-Subproject commit 63adfd8617b40b5939e42329f1b0cffcd56de0c4
+Subproject commit a39c3e8f8ae3b6f31f81d152ecc8f30e4838c09c