Add the products of yecc and leex to the excluded_files list

Including these in the published Hex package makes it not work on
pre-18 versions of Erlang

src/eini.app.src

@@ -6,5 +6,6 @@
     {registered, []},
     {mod, {eini_app, []}},
     {licenses, ["Apache 2.0"]},
-    {links, [{"Github", "https://github.com/erlcloud/eini"}]}
+    {links, [{"Github", "https://github.com/erlcloud/eini"}]},
+    {exclude_files, ["src/eini_parser.erl", "src/eini_lexer.erl"]}
 ]}.