libretro: expose mkLibeRetroCore function

This will allow users to package their own core derivations if they want
without necessary submitting to nixpkgs.

pkgs/misc/emulators/retroarch/cores.nix

@@ -109,6 +109,8 @@ let
     );
 in
 {
+  inherit mkLibRetroCore;
+
   atari800 = mkLibRetroCore {
     core = "atari800";
     description = "Port of Atari800 to libretro";