t0021: fix filehandle usage on older perl

The rot13-filter.pl script calls methods on implicitly
defined filehandles (STDOUT, and the result of an open()
call).  Prior to perl 5.13, these methods are not
automatically loaded, and perl will complain with:

  Can't locate object method "flush" via package "IO::Handle"

Let's explicitly load IO::File (which inherits from
IO::Handle). That's more than we need for just "flush", but
matches what perl has done since:

  http://perl5.git.perl.org/perl.git/commit/15e6cdd91beb4cefae4b65e855d68cf64766965d

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0021/rot13-filter.pl

@@ -21,6 +21,7 @@
 
 use strict;
 use warnings;
+use IO::File;
 
 my $MAX_PACKET_CONTENT_SIZE = 65516;
 my @capabilities            = @ARGV;