Do not handle .d files as object

[taiki-e]

Workaround for #316. Other extensions may need to be added to the ignore list in the future, but it should work as a workaround for the WSL bug for now.

This also replaces cfg!(windows) with more robust ones.

Fixes #316

cc @robo9k Could you test this? (I don't have WSL environment)

[robo9k]

$ cargo +stable install --git https://github.com/taiki-e/cargo-llvm-cov.git --branch object cargo-llvm-cov
    Replaced package `cargo-llvm-cov v0.5.32` with `cargo-llvm-cov v0.5.32 (https://github.com/taiki-e/cargo-llvm-cov.git?branch=object#f40fdb5a)` (executable `cargo-llvm-cov`)

$ cargo +stable llvm-cov --lcov --output-path lcov.info
    Finished report saved to lcov.info

Looks like it's working in WSL with this fix and I'm also seeing coverage in VSCode 🥳

[taiki-e]

Published in 0.5.33.