Add MultiParamTypeClasses pragma to Links example

Add the LANGUAGE pragma 'MultiParamTypeClasses' to the Links example. Otherwise ghc will not compile:

test_links.hs:11:1:
    Illegal instance declaration for `YesodDispatch Links Links'
      (Only one type can be given in an instance head.
       Use -XMultiParamTypeClasses if you want to allow more.)
    In the instance declaration for `YesodDispatch Links Links'