Add logger as a dependency

There was a warning in ruby-head (3.4) that in Ruby 3.5 logger and
ostruct will not be built-in gems, but regular gems.

This change avoids that warning.

The ostruct dependency was be omitted.

Author: olleolleolle

dalli.gemspec

@@ -24,4 +24,6 @@ Gem::Specification.new do |s|
     'changelog_uri' => 'https://github.com/petergoldstein/dalli/blob/main/CHANGELOG.md',
     'rubygems_mfa_required' => 'true'
   }
+
+  s.add_dependency 'logger'
 end

test/helper.rb

@@ -11,7 +11,6 @@
 
 require 'dalli'
 require 'logger'
-require 'ostruct'
 require 'securerandom'
 
 Dalli.logger = Logger.new($stdout)

test/protocol/test_binary.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require 'ostruct'
 require_relative '../helper'
 
 describe Dalli::Protocol::Binary do