Unused parameters in AsyncWinIO.c

Currently the nightly Windows build fails with:

rts\win32\AsyncWinIO.c: In function 'registerAlertableWait':
rts\win32\AsyncWinIO.c:335:69: error:
     error: unused parameter 'num_req' [-Werror=unused-parameter]
      335 | void registerAlertableWait (bool has_timeout, DWORD mssec, uint64_t num_req, bool pending_service)
          |                                                            ~~~~~~~~~^~~~~~~
    |
335 | void registerAlertableWait (bool has_timeout, DWORD mssec, uint64_t num_req, bool pending_service)
    |                                                                     ^
rts\win32\AsyncWinIO.c:335:83: error:
     error: unused parameter 'pending_service' [-Werror=unused-parameter]
      335 | void registerAlertableWait (bool has_timeout, DWORD mssec, uint64_t num_req, bool pending_service)
          |                                                                                   ^
    |
335 | void registerAlertableWait (bool has_timeout, DWORD mssec, uint64_t num_req, bool pending_service)
    |                                                                                   ^
cc1.exe: all warnings being treated as errors

Given that these snuck in to master this presumably means that the validate-x86_64-windows build isn't running with -Werror. I'll need to look into this.