Add missing opus dependency to media/cast/audio_sender/audio_sender.gypi.

Confirmed this fixed the link of the 'chrome' target when uisng the 'remoting=0' flag.

BUG=316483
R=hubbe@chromium.org

Review URL: https://codereview.chromium.org/66913003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233957 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233957