libfreeaptx: split headers to "dev" output

pkgs/development/libraries/libfreeaptx/default.nix

···
     sha256 = "sha256-eEUhOrKqb2hHWanY+knpY9FBEnjkkFTB+x6BZgMBpbo=";
   };
 
+  outputs = [ "out" "dev" ];
+
   postPatch = lib.optionalString stdenv.isDarwin ''
     substituteInPlace Makefile \
       --replace '-soname' '-install_name' \