check statfs and skip

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45666 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
define File::Statfs only if HAVE_STRUCT_STATFS

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45667 b2dd03c8-39d4-4d8f-98ff-823fe69b080e