SDL2_net: move headers to "dev" output

pkgs/development/libraries/SDL2_net/default.nix

···
     sha256 = "sha256-TkqJGYgxYnGXT/TpWF7R73KaEj0iwIvUcxKRedyFf+s=";
   };
 
+  outputs = [ "out" "dev" ];
+
   nativeBuildInputs = [ pkg-config ];
 
   buildInputs = lib.optional stdenv.isDarwin darwin.libobjc;