perl5db.pl: the "i" command must load mro.pm before use

[rjbs]

The i command has apparently been broken either since

commit 8b2b9f856f78ceb0ba1881ae5d55c96b0833425d
Author: Steffen Mueller <smueller@cpan.org>
Date:   Mon Sep 21 16:40:48 2009 +0200

    Remove use of Class::ISA from the debugger

    Instead, use mro::get_linear_isa which accomplishes the same feat. This
    allows us to remove another module from core.

…or since some commit subsequent to that, removing the accidental loading of mro.