protocol: declare when methods override

This avoids compiler warnings about inconsistent use of `override` like the one below.

In file included from /Users/cfriedt/workspace/thrift/test/cpp/src/TestServer.cpp:29:
/Users/cfriedt/workspace/thrift/lib/cpp/src/thrift/protocol/TJSONProtocol.h:255:8: warning: 'checkReadBytesAvailable' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
  void checkReadBytesAvailable(TList& list)
       ^
/Users/cfriedt/workspace/thrift/lib/cpp/src/thrift/protocol/TProtocol.h:563:16: note: overridden virtual function is here
  virtual void checkReadBytesAvailable(TList& list)

PR is here:
https://github.com/apache/thrift/pull/2687

See also:

https://github.com/zephyrproject-rtos/gsoc-2022-thrift/issues/147