Fix warnings:
cmdutils.c: In function ‘opt_default’:
cmdutils.c:304: warning: initialization discards qualifiers from pointer target type
cmdutils.c: In function ‘set_context_opts’:
cmdutils.c:431: warning: passing argument 2 of ‘alloc_priv_context’ discards qualifiers from pointer target type
cmdutils.c:414: note: expected ‘struct AVClass *’ but argument is of type ‘const struct AVClass *’