Fix opus_defines.h to define OPUS_EXPORT only when it's not defined.

Review URL: https://codereview.chromium.org/11260035

git-svn-id: svn://svn.chromium.org/chrome/trunk/deps/third_party/opus@164159 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed