Make CDAudio_Play_byName static.

Make CDAudio_Play_byName static.

It isn't called by any other compilation units, but only internally by
other cd_shared.c functions; more importantly, it does not check for
music_playlist being active like CDAudio_Play does, which would at the
very least be confusing if someone were to add a string version of
qw_svc_cdtrack (WHY WOULD ONE).

Thus: no functional change here.

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@12467 d7cf8633-e32d-0410-b094-e92efae38249