mail/rubygem-mail: Fix dependency with Ruby 3.1

In Ruby 3.1 net/smtp is distributed as bundled gem. On the other hand,
our ruby ports (lang/rubyXY) don't include bundled gems in their
packages and it also applies to lang/ruby31. That means net/smtp isn't
include in the package of Ruby 3.1. So add mail/rubygem-net-smtp to
RUN_DEPENDS so this port works fine with Ruby 3.1.

PR: 262088
Approved by: portmgr (blanket: dependency fix)
MFH: 2022Q1 (dependency fix)