Lua: Don't generate modules/classes for externs.

Externs shouldn't generate any code in the compiled result, but when I
wrote the "type forwarding" that generated modules, etc. at the top of
the script, I forgot to exclude them.

genlua.ml

@@ -1186,9 +1186,11 @@ let generate_type ctx = function
 
 let generate_type_forward ctx = function
 	| TClassDecl c ->
-		generate_package_create ctx c.cl_path;
-		let p = s_path ctx c.cl_path in
-		print ctx "%s = {} " p;
+		if not c.cl_extern then begin
+		    generate_package_create ctx c.cl_path;
+		    let p = s_path ctx c.cl_path in
+		    print ctx "%s = {} " p;
+		end
 	| TEnumDecl e when e.e_extern ->
 		()
 	| TEnumDecl e ->