objxref.pl: improve portability.

crypto/objects/objxref.pl

@@ -39,7 +39,8 @@ my @xrkeys = keys %xref_tbl;
 
 my @srt1 = sort { $oid_tbl{$a} <=> $oid_tbl{$b}} @xrkeys;
 
-for(my $i = 0; $i <= $#srt1; $i++)
+my $i;
+for($i = 0; $i <= $#srt1; $i++)
 	{
 	$xref_tbl{$srt1[$i]}[2] = $i;
 	}