Add an export for the length of a private key

javascript/olm_post.js

@@ -466,6 +466,7 @@ olm_exports["get_library_version"] = restore_stack(function() {
 
 Module['onRuntimeInitialized'] = function() {
     OLM_ERROR = Module['_olm_error']();
+    olm_exports["PRIVATE_KEY_LENGTH"] = Module['_olm_pk_private_key_length']();
     if (onInitSuccess) onInitSuccess();
 };