Fix constness for func[12] parameters in ff_parse_expr() and

Fix constness for func[12] parameters in ff_parse_expr() and
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)’

Originally committed as revision 22860 to svn://svn.ffmpeg.org/ffmpeg/trunk