[Perl/perl5] e7f7ae: Mark vtbl argument in sv_unmagicext const

[Leon Timmermans via perl5-changes]

  Branch: refs/heads/leont/const_unmagic
  Home:   https://github.com/Perl/perl5
  Commit: e7f7ae6e2d787b619e7b2b6876614e8a4ca0b2ed
      
https://github.com/Perl/perl5/commit/e7f7ae6e2d787b619e7b2b6876614e8a4ca0b2ed
  Author: Leon Timmermans <faw...@gmail.com>
  Date:   2023-03-16 (Thu, 16 Mar 2023)

  Changed paths:
    M embed.fnc
    M proto.h
    M sv.c

  Log Message:
  -----------
  Mark vtbl argument in sv_unmagicext const