addToLibraryHeaderSearchPaths - handle non-inherited single values

[grabbou]

Fixes #94. Locally all tests are green, meaning that the change is at least backwards compatible. Not sure if we need any extra test-case for that as the conversion to an array already happens for all $(inherited) values, which is covered.

[Kureev]

Any chance you can take a look on it soon, @alunny? 🙏

[imhotep]

@grabbou any chance you could add a test for single values please ?

[grabbou]

Will do later today. BTW, isn't that already covered by all values = "$(inherited)" ?

[imhotep]

@grabbou is there a test for .push is undefined ?

[grabbou]

Nope since we don't use it anywhere in that method (the error was coming from our code). Now that we convert all string values to an array (not just inherited) https://github.com/alunny/node-xcode/pull/96/files#diff-35bc421794193e73d972308b4f2f3598R1175 it's not going to happen :)