[Perl/perl5] c6b756: Change some USE_ITHREADS to USE_THREADS

[Karl Williamson via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: c6b75602cfb11c134419600556bf1a6428c26d39
      
https://github.com/Perl/perl5/commit/c6b75602cfb11c134419600556bf1a6428c26d39
  Author: Karl Williamson <k...@cpan.org>
  Date:   2024-01-18 (Thu, 18 Jan 2024)

  Changed paths:
    M locale.c
    M makedef.pl
    M perl.h

  Log Message:
  -----------
  Change some USE_ITHREADS to USE_THREADS

These are where the underlying implementation should not matter.