Add missing feature on windows

[jaysonsantos]

If this feature is not enabled, libraries that use keyring would fail with the following message:

error[E0432]: unresolved import `winapi::shared::winerror`
  --> C:\Users\xxxx\scoop\persist\rustup\.cargo\git\checkouts\keyring-rs-b52068140faa8a8e\d4f0c1a\src\windows.rs:10:21
   |
10 | use winapi::shared::winerror::{ERROR_NOT_FOUND, ERROR_NO_SUCH_LOGON_SESSION};
   |                     ^^^^^^^^ could not find `winerror` in `shared`

error[E0432]: unresolved import `winapi::um::errhandlingapi`
  --> C:\Users\xxxx\scoop\persist\rustup\.cargo\git\checkouts\keyring-rs-b52068140faa8a8e\d4f0c1a\src\windows.rs:11:17
   |
11 | use winapi::um::errhandlingapi::GetLastError;
   |                 ^^^^^^^^^^^^^^ could not find `errhandlingapi` in `um`

[jaysonsantos]

Closing this in favor of #58