generator: Don't die if an API call isn't listed in API versions file.

For example, it may just have been added.

generator/generator_api_versions.ml

@@ -39,6 +39,5 @@ let load_api_versions filename =
   close_in chan
 
 let lookup_api_version sym =
-  try Hashtbl.find hash sym
-  with Not_found ->
-    failwithf "API symbol \"%s\" not found in API versions file" sym
+  try Some (Hashtbl.find hash sym)
+  with Not_found -> None

generator/generator_api_versions.mli

@@ -25,7 +25,8 @@
 val load_api_versions : string -> unit
 (** Load the data from the named file. *)
 
-val lookup_api_version : string -> string
+val lookup_api_version : string -> string option
 (** [lookup_api_version c_api] looks up the version that the C API call
     (which must be the full C name, eg. ["guestfs_launch"]) was
-    added.  This returns the version string, eg. ["0.3"]. *)
+    added.  This returns the version string, eg. [Some "0.3"], or
+    [None] if no version could be found. *)

generator/generator_c.ml

@@ -255,8 +255,10 @@ L</KEYS AND PASSPHRASES> for more information.\n\n";
          | None -> ()
          | Some txt -> pr "%s\n\n" txt
         );
-        let version = lookup_api_version name in
-        pr "(Added in %s)\n\n" version;
+        (match lookup_api_version name with
+         | Some version -> pr "(Added in %s)\n\n" version
+         | None -> ()
+        );
 
         (* Handling of optional argument variants. *)
         if optargs <> [] then (