build: allow cargo-tarpaulin config

[PsypherPunk]

As per Clippy's own recommendation:

161 | #[cfg(not(tarpaulin_include))]
    |           ^^^^^^^^^^^^^^^^^
    |
    = help: consider using a Cargo feature instead
    = help: or consider adding in `Cargo.toml` the `check-cfg` lint config for the lint:
             [lints.rust]
             unexpected_cfgs = { level = "warn", check-cfg = ['cfg(tarpaulin_include)'] }
    = help: or consider adding `println!("cargo::rustc-check-cfg=cfg(tarpaulin_include)");` to the top of the `build.rs`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration

fixes #646