Fix for Mach-O symbol extraction
The `n_sect` field is 1-based with `0` meaning `NO_SECT` meaning the
symbol is not present in any section in the library. This change fixes
the index mismatch.
Reference: https://github.com/aidansteele/osx-abi-macho-file-format-reference#nlist_64,
The code was mostly working before because of the way we calculate the
offset (https://github.com/mozilla/uniffi-rs/blob/2e3b59cf6dbcb46983028b08afc6e0ee51183f2e/uniffi_bindgen/src/macro_metadata/extract.rs#L106). Because of this, even if we were off by one when picking the section, things would still work as long as the two sections involved lined up correctly.
Thanks to 0c0w3 for pointing this out and sending me the patch.
