rubocop: exclude lib/fluent/config/dsl.rb in Security/Eval

[Watson1978]

Which issue(s) this PR fixes:
Fixes #

What this PR does / why we need it:
Now, lib/fluent/config/dsl.rb causes failure on rubocop check.

Related to https://github.com/fluent/fluentd/actions/runs/16310944121/job/46066401079

Kernel.eval is used to get a constant value. There are no security issues.

fluentd/lib/fluent/config/dsl.rb

Lines 132 to 140 in 41d5102

	def self.const_missing(name)
	return ::Kernel.const_get(name) if ::Kernel.const_defined?(name)
	if name.to_s =~ /^Fluent::Config::DSL::Element::(.*)$/
	name = "#{$1}".to_sym
	return ::Kernel.const_get(name) if ::Kernel.const_defined?(name)
	end
	::Kernel.eval("#{name}")
	end

So, this PR will excude the file.

Docs Changes:
Not needed.

Release Note:

[daipom]

Thanks!
Could you briefly describe why it is a false positive?

[Watson1978]

Could you briefly describe why it is a false positive?

Update the description.
Thanks.

[daipom]

LGTM! Thanks!