fiddle/import.rb: fix typo

* ext/fiddle/lib/fiddle/import.rb (Fiddle::Importer#sizeof): fix typo,
  SIZEOF_LONG_LON.  [Fix GH-714]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47590 b2dd03c8-39d4-4d8f-98ff-823fe69b080e