Add \sqrt[4] as Latex input method, too

* lisp/leim/quail/latin-ltx.el: Add \sqrt[4] as input method
(bug#25594).

Author: larsmagne

lisp/leim/quail/latin-ltx.el

@@ -705,6 +705,7 @@ system, including many technical ones.  Examples:
  ("\\defs" ?≙)				; per fuzz/zed
  ("\\sqrt" ?√)
  ("\\sqrt[3]" ?∛)
+ ("\\sqrt[4]" ?∜)
  ("\\llbracket" ?\〚) 			; stmaryrd
  ("\\rrbracket" ?\〛)
  ;; ("\\lbag" ?\〚) 			; fuzz