[Perl/perl5] 1892bf: Suppress -Wunused_variable warning during 'make'

[James E Keenan via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: 1892bf8c3ca0bdfe5f0667cb756fcaeb6b2312b8
      
https://github.com/Perl/perl5/commit/1892bf8c3ca0bdfe5f0667cb756fcaeb6b2312b8
  Author: James E Keenan <jkee...@cpan.org>
  Date:   2023-08-21 (Mon, 21 Aug 2023)

  Changed paths:
    M pp_ctl.c

  Log Message:
  -----------
  Suppress -Wunused_variable warning during 'make'

Implement Dave Mitchell's suggestion: we don't really need the unused
variable.

For: https://github.com/Perl/perl5/issues/21393