Add base64 gem as dependency for Ruby 3.4

The `base64` gem was marked as bundled gem for Ruby 3.4.
Ref. ruby/ruby@6500f85

This patch will suppress the following warning message:

```
socket_manager.rb:23: warning: base64 was loaded from the standard library, but will no longer be part of the default gems since Ruby 3.4.0. Add base64 to your Gemfile or gemspec.
```

Signed-off-by: Shizuo Fujita <fujita@clear-code.com>

serverengine.gemspec

@@ -19,6 +19,7 @@ Gem::Specification.new do |gem|
   gem.required_ruby_version = ">= 2.3.0"
 
   gem.add_dependency "sigdump", ["~> 0.2.2"]
+  gem.add_dependency "base64", ["~> 0.1"]
 
   # rake v12.x doesn't work with rspec 2. rspec should be updated to 3
   gem.add_development_dependency "rake", ["~> 13.0"]