move libid3tag's pkgconfig file into its nix-expression directory

Hi,

I recently made a change to libid3tag https://github.com/NixOS/nixpkgs/blob/2d31046a40e2ae5546971728477a0a1a17c49e08/pkgs/development/libraries/libid3tag/default.nix#L17-L35. I didn't know that you could reference files in the directory of the current nix-expression. I think that makes a little bit prettier.

Best,
Sven