[Perl/perl5] c31ae9: Rename variable in EXTEND_HWM_SET

[Hugo van der Sanden via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: c31ae9a243a483ee972d34e51cfaca9525f50d05
      
https://github.com/Perl/perl5/commit/c31ae9a243a483ee972d34e51cfaca9525f50d05
  Author: Hugo van der Sanden <h...@crypt.org>
  Date:   2022-06-06 (Mon, 06 Jun 2022)

  Changed paths:
    M pp.h

  Log Message:
  -----------
  Rename variable in EXTEND_HWM_SET

"ix" is a common variable name in XS code, and may be passed in as
part of a parameter.