[Perl/perl5] fe5892: fix PERL_GLOBAL_STRUCT_PRIVATE builds

[iabyn]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: fe5892c19249419cf5c19730bf44e2dd7703be46
      
https://github.com/Perl/perl5/commit/fe5892c19249419cf5c19730bf44e2dd7703be46
  Author: David Mitchell <da...@iabyn.com>
  Date:   2020-03-20 (Fri, 20 Mar 2020)

  Changed paths:
    M op.c
    M util.c

  Log Message:
  -----------
  fix PERL_GLOBAL_STRUCT_PRIVATE builds

Sprinkle some dVAR pixie dust to fix recent bitrot on that build
configuration.