RFR: 7903239: ofAddress factory of function pointer type is wrong for struct returns

[Marko]

When generating the lambda inside the `ofAddress` factory, it is missing the `SegmentAllocator` needed when the function returns a struct. This is [implemented for regular functions](https://github.com/openjdk/jextract/blob/0582eaf1b4cdba95f0ee8c2480767433bb647d0d/src/main/java/org/openjdk/jextract/impl/HeaderFileBuilder.java#L122-L127), but not for function pointers.

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Commit messages:
 - 7903239: ofAddress factory of function pointer type is wrong for struct returns

Changes: https://git.openjdk.org/jextract/pull/58/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=58&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903239
  Stats: 47 lines in 3 files changed: 44 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jextract/pull/58.diff
  Fetch: git fetch https://git.openjdk.org/jextract pull/58/head:pull/58

PR: https://git.openjdk.org/jextract/pull/58