Add missing Closure extern for ftruncateSync (#14877)

Otherwise, the Closure Compiler might rename fs.ftruncateSync
during compilation when the NODERAWFS filesystem backend
is used.

tests/test_core.py

@@ -5382,6 +5382,7 @@ Module['onRuntimeInitialized'] = function() {
   def test_unistd_truncate_noderawfs(self):
     self.uses_es6 = True
     self.set_setting('NODERAWFS')
+    self.maybe_closure()
     self.do_run_in_out_file_test('unistd/truncate.c', js_engines=[config.NODE_JS])
 
   def test_unistd_swab(self):

third_party/closure-compiler/node-externs/fs.js

@@ -56,6 +56,12 @@ fs.truncate = function(fd, len, callback) {};
  */
 fs.truncateSync = function(fd, len) {};
 
+/**
+ * @param {*} fd
+ * @param {number} len
+ */
+fs.ftruncateSync = function(fd, len) {};
+
 /**
  * @param {string} path
  * @param {number} uid