Add mapping to Cabal extra-libraries: bcrypt (#1561)

https://docs.microsoft.com/en-us/windows/win32/api/bcrypt/

N.B. The `BCryptGenRandom` symbol is provided by MinGW `bcrypt.h` and needed by
https://github.com/yvan-sraka/greetings (and I guess any project relying on
Rust `std`) when cross-compiling to `x86_64-w64-mingw32`

lib/system-nixpkgs-map.nix

@@ -129,6 +129,7 @@ in
      msvcrt = null; # this is the libc
      Crypt32 = null;
      mswsock = null;
+     bcrypt = null;
    }
 # -- os x
 # NB: these map almost 1:1 to the framework names