Add src/ to include_dirs in erlang_ls.config

So that ErlangLS can find typechecker.hrl, too.
Otherwise, it returned:

```
[2023-06-16T21:26:14.858108+02:00] [warning] Failed to find the following include files for "/Users/erszcz/work/erszcz/gradualizer/test/absform_tests.erl":, ["typechecker.hrl"] [ L] <0.131.0>
[2023-06-16T21:26:14.918620+02:00] [warning] Failed to find the following include files for "/Users/erszcz/work/erszcz/gradualizer/test/typechecker_tests.erl":, ["typechecker.hrl"] [ L] <0.131.0>
[2023-06-16T21:26:14.923993+02:00] [warning] Failed to find the following include files for "/Users/erszcz/work/erszcz/gradualizer/test/typelib_tests.erl":, ["typelib.hrl"] [ L] <0.131.0>
```

erlang_ls.config

@@ -7,6 +7,7 @@ code_path_extra_dirs:
   - "_build/test/lib/gradualizer/test/property_test"
 include_dirs:
   - "include"
+  - "src"
   - "_build/default/lib/"
 diagnostics:
   enabled: