src: add override to ThreadPool methods in zlib

[danbev]

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.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

[danbev]

CI: https://ci.nodejs.org/job/node-test-pull-request/14903/

[addaleax]

LGTM

Not sure if my compiler just doesn’t display these or I’m overlooking them 🙃

[bnoordhuis]

@addaleax You use gcc < 5? -Winconsistent-missing-override is a clang special. gcc calls it -Wsuggest-override and (I think) turns it on with -Wall.

[addaleax]

7.2.0, actually. It does provide warnings when used with -Wsuggest-override, but that suggests override for other spots in the source too.

[danbev]

Would it be alright to fast-track this issue?

[danbev]

Landed in 0419adc.