Load 'mini_portile2' only when needed.

This enables installation using system SQLite3 without having
mini_portile2 around:

~~~
gem install --force sqlite3 -- --enable-system-libraries
~~~

ext/sqlite3/extconf.rb

@@ -1,5 +1,4 @@
 require "mkmf"
-require "mini_portile2"
 require "yaml"
 
 module Sqlite3
@@ -131,6 +130,8 @@ def configure_extension
       end
 
       def minimal_recipe
+        require "mini_portile2"
+
         MiniPortile.new(libname, sqlite3_config[:version]).tap do |recipe|
           if sqlite_source_dir
             recipe.source_directory = sqlite_source_dir