made SML/NJ happy;

--- a/src/HOL/Nominal/nominal_induct.ML	Wed Jan 13 23:37:55 2016 +0100
+++ b/src/HOL/Nominal/nominal_induct.ML	Thu Jan 14 12:11:22 2016 +0100
@@ -165,7 +165,7 @@
 
 in
 
-val nominal_induct_method =
+val nominal_induct_method : (Proof.context -> Proof.method) context_parser =
   Scan.lift (Args.mode Induct.no_simpN) --
   (Parse.and_list' (Scan.repeat (unless_more_args def_inst)) --
     avoiding -- fixing -- rule_spec) >>