Fix leveldb compilation for NetBSD

Summary:
Backport of core PR12294
https://github.com/bitcoin/bitcoin/pull/12294/files

This backport has not been tested and does not include the
documentation, as there is no demand for adding support for NetBSD.
The only purpose is to help doing so if want to do it in the future.

Test Plan:

make
ninja

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3816