Include bzlib.h in libbz2-rs-sys

[j-vanderstoep]

Symlink it to bzlib.h in the parent crate.

This needs to be included for direct users of this crate.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
fuzz-compress_then_decompress	?
fuzz-decompress_chunked	?
test-aarch64-apple-darwin	89.49% <ø> (ø)
test-x86_64-apple-darwin	89.49% <ø> (ø)
test-x86_64-unknown-linux-gnu	89.49% <ø> (ø)

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see 2 files with indirect coverage changes

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[folkertdev]

Nice, thanks!