mingw-w64: Depend on own headers derivation

Without this, a `#include <float.h>` resolves incorrectly. Either the
headers weren't on the include path at all, or they only were for
local, not system, imports.

What's weird is this used to not be a problem. Not sure what other
change in e.g. cc-wrapper would affect this.

pkgs/os-specific/windows/mingw-w64/default.nix

@@ -1,6 +1,7 @@
-{ stdenv, callPackage }:
+{ stdenv, callPackage, windows }:
 
 stdenv.mkDerivation {
   inherit (callPackage ./common.nix {}) name src;
+  buildInputs = [ windows.mingw_w64_headers ];
   dontStrip = true;
 }