lib/webrick.rb: use require_relative for webrick/utils.rb

`require "webrick/utils.rb"` should load `lib/webrick/utils.rb`, but on
some CI machines, it wrongly loads `test/webrick/utils.rb`, which caused
circular require warning:

https://rubyci.org/logs/rubyci.s3.amazonaws.com/ubuntu/ruby-master/log/20190825T093007Z.log.html.gz
```
/data/chkbuild/tmp/build/20190825T093007Z/ruby/lib/rubygems/core_ext/kernel_require.rb:61: warning: /data/chkbuild/tmp/build/20190825T093007Z/ruby/lib/rubygems/core_ext/kernel_require.rb:61: warning: loading in progress, circular require considered harmful - /data/chkbuild/tmp/build/20190825T093007Z/ruby/lib/webrick.rb
```

lib/webrick.rb

@@ -212,7 +212,7 @@ require 'webrick/version.rb'
 require 'webrick/config.rb'
 require 'webrick/log.rb'
 require 'webrick/server.rb'
-require 'webrick/utils.rb'
+require_relative 'webrick/utils.rb'
 require 'webrick/accesslog'
 
 require 'webrick/htmlutils.rb'