oggdec internationalization is broken

While trying to fix a different Debian bug in "oggdec", I've discovered that the "oggdec" executable uses the "_" (underscore) macro for internationalization and has translations of the relevant strings available in the .po files, but nevertheless outputs everything in English. This is probably due to a lack of initialization of the internationalization modules in that executable. I'm contributing a patch that adds the necessary initializations (copied from the other executables in vorbis-tools).