Adapt map_term_with_context for 9.1 guard condition

[BasileGros]

Adds back a change lost in the confusion of the transfer from 9.0 to 9.1 of an adaptation of map_term_with_context to work with the 9.1 guard condition.

Previous version does not allow to define in 9.1 the following MWE function:

Fixpoint convert_var_to_rel (glob : global_env)  (lctx : context) (t:term) {struct t} :=
  match t with
  |tVar _ => tRel (S (length lctx))
  |_ => map_term_with_context glob (convert_var_to_rel glob) lctx t
  end.