This change will avoid exporting libvpx symbols if libvpx is not built into
FFmpeg.

BUG=None
TEST=FFmpeg can be loaded on ARM

Review URL: http://codereview.chromium.org/3518002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@61013 0039d316-1c4b-4281-b951-d872f2087c98