src: add override to ThreadPool methods in zlib

Currently the following compiler warnings are generated:
../src/node_zlib.cc:222:8:
warning: 'DoThreadPoolWork' overrides a member function but is not marked
      'override' [-Winconsistent-missing-override]
  void DoThreadPoolWork() {
       ^
../src/node_internals.h:509:16: note: overridden virtual function is here
  virtual void DoThreadPoolWork() = 0;
               ^
../src/node_zlib.cc:357:8: warning:
'AfterThreadPoolWork' overrides a member function but is not marked
      'override' [-Winconsistent-missing-override]
  void AfterThreadPoolWork(int status) {
       ^
../src/node_internals.h:510:16:
note: overridden virtual function is here
  virtual void AfterThreadPoolWork(int status) = 0;
               ^

This commit adds the override specifier to the methods to silence the
warnings.

PR-URL: #20769
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

src/node_zlib.cc

@@ -219,7 +219,7 @@ class ZCtx : public AsyncWrap, public ThreadPoolWork {
   // This function may be called multiple times on the uv_work pool
   // for a single write() call, until all of the input bytes have
   // been consumed.
-  void DoThreadPoolWork() {
+  void DoThreadPoolWork() override {
     const Bytef* next_expected_header_byte = nullptr;
 
     // If the avail_out is left at 0, then it means that it ran out
@@ -353,7 +353,7 @@ class ZCtx : public AsyncWrap, public ThreadPoolWork {
 
 
   // v8 land!
-  void AfterThreadPoolWork(int status) {
+  void AfterThreadPoolWork(int status) override {
     write_in_progress_ = false;
 
     if (status == UV_ECANCELED) {