hide "redefine" warnings

This is necesary in the odd case where a user has code importing from another
file that is not in a package.  IE:

  foo.pl
  --------------------------
  use Log::Contextual ':log';
  require 'bar.plx';

  log_debug { 'test' };
  --------------------------

  bar.plx
  --------------------------
  use Log::Contextual ':log', 'set_logger';
  use Log::Contextual::SimpleLogger;
  set_logger(
     Log::Contextual::SimpleLogger->new({ levels => [qw( debug )]})
  );
  1;
  --------------------------

In this code without the warning removed, the user gets a warning no matter
what.