ff_parse_and_eval_expr().

Change func[12] attributes from "** func" to "* const * func".

This is consistent with the semantics of the provided arrays of
functions, which are not supposed to be changed by the ff_parse_*
functions.

Also fix the GCC compilation warnings:
libavcodec/ratecontrol.c: In function ‘ff_rate_control_init’:
libavcodec/ratecontrol.c:109: warning: passing argument 3 of ‘ff_parse_expr’ discards qualifiers from pointer target type
libavcodec/eval.h:69: note: expected ‘double (**)(void *, double)’ but argument is of type ‘double (* const*)(void *, double)’

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@22860 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b