[Perl/perl5] ffb1f0: Make argument to S_is_invlist pointer to const

[Dagfinn Ilmari Mannsåker via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: ffb1f0fda2556c3e6d57b8a57bbe7ea9292316f1
      
https://github.com/Perl/perl5/commit/ffb1f0fda2556c3e6d57b8a57bbe7ea9292316f1
  Author: Dagfinn Ilmari Mannsåker <ilm...@ilmari.org>
  Date:   2022-04-13 (Wed, 13 Apr 2022)

  Changed paths:
    M embed.fnc
    M invlist_inline.h
    M proto.h
    M regcomp.c

  Log Message:
  -----------
  Make argument to S_is_invlist pointer to const

This lets us avoid casting away the const in S_invlist_max()