Add explicit require for node_factory

This resolves an error that is caused by the removal of `#sort`
from Dir.glob. Dir.glob produces a slightly different file ordering
without sort and thus causes the below missing constant error.

```
Failure/Error:
  def allow_extensions(regex: EXTENSION_REGEX, &block)
    @extension_regex = regex
    @allowed_extensions = block || true
  end

NameError:
  uninitialized constant Openapi3Parser::NodeFactory::ObjectFactory::Dsl::EXTENSION_REGEX

          def allow_extensions(regex: EXTENSION_REGEX, &block)
```

Author: kevindew

lib/openapi3_parser/node_factory/object_factory/dsl.rb

@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require "openapi3_parser/node_factory"
 require "openapi3_parser/node_factory/object_factory/field_config"
 
 module Openapi3Parser