Update common submodule

This pulls in the commits below, requiring us to replace all uses of
String.is_prefix and String.is_suffix.

Mostly done with Perl like this, and carefully checked by hand
afterwards since this doesn't get everything right:

  $ perl -pi.bak -e 's/String.is_prefix ([^[:space:]\)]+) ([^[:space:]\)]+)/String.starts_with \2 \1/g' -- `git ls-files`

  Richard W.M. Jones (3):
      mlstdutils: Fix comment that still referred to the old function names
      mldrivers: Link to gettext-stub if ocaml-gettext is enabled
      mlstdutils: Rename String.is_prefix -> starts_with, is_suffix -> ends_with

generator/daemon.ml

@@ -511,7 +511,7 @@ let rec generate_daemon_caml_interface modname () =
   generate_header OCamlStyle GPLv2plus;
 
   let is_ocaml_module_function = function
-    | { impl = OCaml m } when String.is_prefix m (modname ^ ".") -> true
+    | { impl = OCaml m } when String.starts_with (modname ^ ".") m -> true
     | { impl = OCaml _ } -> false
     | { impl = C } -> false
   in