Build error on window caused by unique_ptr

[yrliou]

In file included from ../../brave/vendor/bat-native-ledger/src/bat_state.cc:5:
../../brave/vendor/bat-native-ledger/src/bat_state.h(140,8):  error: no template                                                    named 'unique_ptr' in namespace 'std'
  std::unique_ptr<braveledger_bat_helper::CLIENT_STATE_ST> state_;
  ~~~~~^
../../brave/vendor/bat-native-ledger/src/bat_state.h(140,60):  error: private fi                                                   eld 'state_' is not used [-Werror,-Wunused-private-field]
  std::unique_ptr<braveledger_bat_helper::CLIENT_STATE_ST> state_;
                                                           ^
2 errors generated.

This is introduced by brave-intl/bat-native-ledger#171, so should be in the same milestone as #1997

[bbondy]

Adjusting milestone after it was uplifted to 0.57.x.