Avoid using incompatible pointer type for `old_warn`

[chrstphrchvz]

See https://github.com/eserte/perl-tk/issues/98#issuecomment-1944054296: Newer compilers may have -Werror=incompatible-pointer-types by default. PL_curcop->cop_warnings previously had type SV *, then STRLEN *, and now char * as of Perl 5.37.6. Since old_warn is only used to temporarily hold the value from PL_curcop->cop_warnings without dereferencing it, old_warn can instead be declared as void * regardless of Perl 5 version.