winsvc: require serverengine

In #3849, getting ruby's path was replaced with
`ServerEngine.ruby_bin_path` but `require 'serverengine'` was missing.
As a result it fails to launch fluentd service.

Signed-off-by: Takuro Ashie <ashie@clear-code.com>

lib/fluent/winsvc.rb

@@ -19,6 +19,8 @@
   require 'optparse'
   require 'win32/daemon'
   require 'win32/event'
+  require 'win32/registry'
+  require 'serverengine'
 
   include Win32
 
@@ -33,7 +35,6 @@
   end 
 
   def read_fluentdopt(service_name)
-    require 'win32/registry'
     Win32::Registry::HKEY_LOCAL_MACHINE.open("SYSTEM\\CurrentControlSet\\Services\\#{service_name}") do |reg|
       reg.read("fluentdopt")[1] rescue ""
     end