parser_multiline.rb must require parser_regexp in order to use Fluent…

…::Plugin::RegexpParser

Because parser_multiline uses Fluent::Plugin::RegexpParser it must
require parser_regexp, otherwise, you get error messages like this
when using in_tail with format multiline:
```
2017-02-09 02:02:25 +0000 [error]: #0 fluent/supervisor.rb:674:block in main_process: config error file="/etc/fluent/fluent.conf" error_class=Fluent::ConfigError error="Invalid regexp '^(?<time>\\S+\\s+\\S+\\s+\\S+)\\s+(?<host>\\S+)\\s+(?<ident>[\\w\\/\\.\\-]*)(?:\\[(?<pid>[0-9]+)\\])?[^\\:]*\\:\\s*(?<message>.*)$': uninitialized constant Fluent::Plugin::RegexpParser\nDid you mean?  RegexpError"
```

lib/fluent/plugin/parser_multiline.rb

@@ -15,6 +15,7 @@
 #
 
 require 'fluent/plugin/parser'
+require 'fluent/plugin/parser_regexp'
 
 module Fluent
   module Plugin
@@ -29,13 +30,17 @@ def configure(conf)
         super
 
         formats = parse_formats(conf).compact.map { |f| f[1..-2] }.join
+        $log.error("MultilineParser.configure #{conf} formats orig #{parse_formats(conf).compact} join #{formats}")
         begin
           regexp = Regexp.new(formats, Regexp::MULTILINE)
+          $log.error("MultilineParser.configure regexp #{regexp}")
           if regexp.named_captures.empty?
             raise "No named captures"
           end
           regexp_conf = Fluent::Config::Element.new("", "", { "expression" => "/#{formats}/", "multiline" => true }, [])
+          $log.error("MultilineParser.configure regexp_conf #{regexp_conf}")
           @parser = Fluent::Plugin::RegexpParser.new
+          $log.error("MultilineParser.configure parser #{@parser}")
           @parser.configure(conf + regexp_conf)
         rescue => e
           raise Fluent::ConfigError, "Invalid regexp '#{formats}': #{e}"