pin camlp5 to 8.0-alpha05 for now

opam

@@ -22,7 +22,7 @@ depends: [
   "ocaml" {>= "4.02"}
   "ocamlfind" {build}
   "dune" {>= "1.11"}
-  "camlp5" {build}
+  "camlp5" {build & = "8.00~alpha05"}
   "sedlex" {>= "2.0"}
   "xml-light"
   "extlib" {>= "1.7.6"}