[Perl/perl5] a5fea3: locale.c: Add missing mutex locks

[Karl Williamson via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: a5fea3d62850346b762a77145cd0318378c48d2d
      
https://github.com/Perl/perl5/commit/a5fea3d62850346b762a77145cd0318378c48d2d
  Author: Karl Williamson <k...@cpan.org>
  Date:   2023-07-17 (Mon, 17 Jul 2023)

  Changed paths:
    M locale.c

  Log Message:
  -----------
  locale.c: Add missing mutex locks

This adds locks around mbrtowc and reading the environment that a code
review indicated were missing.  The mbrtowc lock is currently a no-op,
but a future commit will change that.  The other lock is in code rarely
compiled.