[Perl/perl5] adc48c: perl.h: Add EXTERN_C for flock()

[Karl Williamson]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: adc48c40662e4c147b1f9b171574d14903affb00
      
https://github.com/Perl/perl5/commit/adc48c40662e4c147b1f9b171574d14903affb00
  Author: Karl Williamson <k...@cpan.org>
  Date:   2020-01-21 (Tue, 21 Jan 2020)

  Changed paths:
    M perl.h

  Log Message:
  -----------
  perl.h: Add EXTERN_C for flock()

This was failing to load on Solaris with g++

Thanks to Dagfinn Ilmari Mannsåker for figuring this out.