Fix uninitialized constant error by requiring lib/security.rb in lib/…

…parsers/baseparser.rb

This commit fixes an issue in `REXML::Parsers::BaseParser`
where the `Security` module is not initialized, causing a NameError.

How to reproduce

When `lib/rexml/parsers/baseparser.rb` is loaded directly,
the `Security` module is not required, leading to the following error.

```ruby
require_relative "./lib/rexml/parsers/baseparser"

pp REXML::Security.name
```
```console
$ ruby test.rb
test.rb:3:in `<main>': uninitialized constant REXML::Security (NameError)

pp REXML::Security.name
        ^^^^^^^^^^
Did you mean?  SecurityError
```

How to resolve

Require `Security` module in `lib/rexml/parsers/baseparser.rb` to
ensure the `Security` module is initialized when `BaseParser` is loaded.

lib/rexml/parsers/baseparser.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 require_relative '../parseexception'
 require_relative '../undefinednamespaceexception'
+require_relative '../security'
 require_relative '../source'
 require 'set'
 require "strscan"