Normalize path output for --dump-includes

[xpple]

This PR normalizes the path output for --dump-includes.

1. Currently, when using jextract with multiple header files, the path outputs for --dump-includes may not be normalized. For example, the output could be:

   #### Extracted from: [...]/mylib/dir/../header.h
   
   --include-constant CONSTANT # header: [...]/mylib/dir/../header.h
   

2. This is problematic because one may wish to use tools like grep to filter based on known paths to header files.
3. To resolve this, I simply added a normalize() call in IncludeHelper.java.

I didn't test the PR locally using jtreg, but the tests on GitHub Actions ran successfully. I did also confirm that the new output correctly normalizes each path:

#### Extracted from: [...]/mylib/header.h

--include-constant CONSTANT # header: [...]/mylib/header.h

---

Progress

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Reviewing

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Checkout this PR locally:
$ git fetch https://git.openjdk.org/jextract.git pull/307/head:pull/307
$ git checkout pull/307

Update a local copy of the PR:
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$ git pr checkout 307

View PR using the GUI difftool:
$ git pr show -t 307

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Download this PR as a diff file:
https://git.openjdk.org/jextract/pull/307.diff

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[openjdk]

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

[xpple]

I am not able to sign the OCA:

Since it's only a one line change, the change can probably be integrated through means other than this PR.