lavf/matroskaenc: Fix Cues on Audio for WebM DASH

Fix a change that was broken by [1]. Cues must be added for audio frames
on cluster start for WebM when the DASH flag is passed. Restoring
correct functionality.

[1] http://goo.gl/xYLq7Z

Signed-off-by: Vignesh Venkatasubramanian <vigneshv@google.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>