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.

Perhaps the ostruct dependency can be ommitted, I found no usages of it
in the source code of dalli.

Gemfile

@@ -6,6 +6,7 @@ gemspec
 
 group :development, :test do
  gem 'connection_pool'
+ gem "ostruct" # Avoids warning: ostruct was loaded from the standard library, but will no longer be part of the default gems since Ruby 3.5.0.
  gem 'minitest', '~> 5'
  gem 'rack', '~> 2.0', '>= 2.2.0'
  gem 'rake', '~> 13.0'

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