Fix org-babel-header-args:haskell not being an alist

* lisp/ob-haskell.el (org-babel-header-args:haskell): The old value of
the variable was not argument alist, as expected by Org babel code.

Fixes https://orgmode.org/list/87fsty7750.fsf@gmail.com

lisp/ob-haskell.el

@@ -70,7 +70,7 @@ a parameter, such as \"ghc -v\"."
   :package-version '(Org "9.4")
   :type 'string)
 
-(defconst org-babel-header-args:haskell '(compile . :any)
+(defconst org-babel-header-args:haskell '((compile . :any))
   "Haskell-specific header arguments.")
 
 (defun org-babel-haskell-execute (body params)