hb_rate_s, hb_mixdown_s, hb_encoder_s: use const char * instead of char *

These are used in hardcoded tables, so there will never be a need for mutable strings.



git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5148 b64f7644-9d1e-0410-96f1-a4d463321fa5