Use Gem::PackageTask instead of Rake one

Rake::GemPackageTask is deprecated and will be removed.

Gem::PackageTask has been present since RubyGems 1.3.2 and should be
used instead.

rake-compiler already depends on rubygems >= 1.3.5 so no damage done here.

lib/rake/baseextensiontask.rb

@@ -14,8 +14,6 @@
 require 'pathname'
 
 module Rake
-  autoload :GemPackageTask, 'rake/gempackagetask'
-
   class BaseExtensionTask < TaskLib
 
     attr_accessor :name

lib/rake/extensiontask.rb

@@ -1,6 +1,7 @@
 #!/usr/bin/env ruby
 
 require 'rake/baseextensiontask'
+require "rubygems/package_task"
 
 # Define a series of tasks to aid in the compilation of C extensions for
 # gem developer/creators.
@@ -213,7 +214,7 @@ def define_native_tasks(for_platform = nil, ruby_ver = RUBY_VERSION, callback =
           end
 
           # Generate a package for this gem
-          gem_package = Rake::GemPackageTask.new(spec) do |pkg|
+          Gem::PackageTask.new(spec) do |pkg|
             pkg.need_zip = false
             pkg.need_tar = false
           end