Add mapping to Cabal extra-libraries: bcrypt

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`

Author: yvan-sraka

lib/system-nixpkgs-map.nix

@@ -18,7 +18,9 @@ let
   ] else [];
 in
 # -- linux
-{ crypto = [ openssl ];
+{
+  bcrypt = [ ];
+  crypto = [ openssl ];
   "c++" = []; # no libc++
   "stdc++" = gcclibs;
   "stdc++-6" = gcclibs;