Fix opus.h for doxygen when referencing alternative values.

Closed Alexander Traud requested to merge traud/opus:traud-master-patch-69816 into master

The latest Doxygen was not able to resolve the references because it looked for OPUS_APPLICATION_VOIP/@ref. Because the second reference could not be resolved either, Doxygen printed two warnings actually:

include/opus.h:201: warning: unable to resolve reference to 'OPUS_APPLICATION_VOIP/@ref' for \ref command
include/opus.h:201: warning: expected whitespace after \ref command

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