Add dummy 4.08 grammar, equal to the 4.07 one

Author: NathanReb

ast/builder.ml

@@ -1,3 +1,8 @@
+module V4_08 = struct
+  include Builder_common
+  include Builder_v4_08
+end
+
 module V4_07 = struct
   include Builder_common
   include Builder_v4_07

ast/builder.mli

@@ -1,3 +1,8 @@
+module V4_08 : sig
+  include module type of Builder_common
+  include module type of Builder_v4_08
+end
+
 module V4_07 : sig
   include module type of Builder_common
   include module type of Builder_v4_07