barectf.c.j2: do not pass useless parameters to _ev_size_*() functions

barectf.c.j2: do not pass useless parameters to _ev_size_*() functions

This patch changes _CodeGen._proto_params_str() in `gen.py` to accept a
`only_dyn` parameter. When `only_dyn` is `True`, then the function only
generates function prototype parameter strings for field types which
have a dynamic size (string field types, as of this version).

This patch also changes the ft_call_params() macro to add an `only_dyn`
parameter which serves the same purpose.

The purpose of this is not calling the _ev_size_*() functions with
parameters which correspond to statically-sized field types.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>