nixos/fontconfig: add missing config for Xft.hintstyle

This config is removed when removing[1] fonts.fontconfig.hinting.style
option.

However, when adding[2] that option back, this config is missing.

[1]: https://github.com/NixOS/nixpkgs/commit/65592837b6e62fb555d6e8c891f347428886c4f2
[2]: https://github.com/NixOS/nixpkgs/commit/659096dd89da4419200b917567f97906805b3131

nixos/modules/services/x11/display-managers/default.nix

@@ -24,7 +24,7 @@ let
     Xft.lcdfilter: lcd${fontconfig.subpixel.lcdfilter}
     Xft.hinting: ${if fontconfig.hinting.enable then "1" else "0"}
     Xft.autohint: ${if fontconfig.hinting.autohint then "1" else "0"}
-    Xft.hintstyle: hintslight
+    Xft.hintstyle: ${fontconfig.hinting.style}
   '';
 
   # file provided by services.xserver.displayManager.sessionData.wrapper