build: fix freebsd support

[6xd7]

Description

30d9410 removed FreeBSD support added in 666b412

This pr re-add FreeBSD as a target_os in build.rs

Motivation and Context

Broken build in FreeBSD

How Has This Been Tested?

cargo build --release

What process can a PR reviewer use to test or verify this change?

Breaking Changes

• None
• Requires data directory on base node to be deleted
• Requires hard fork
• Other - Please specify

[github-actions]

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[hansieodendaal]

I checked the Windows build locally, which did not give any problems, however, we have some clippy issues: cargo lints clippy --all-targets --all-features.