Normalize header path to use unix sep
Normalize header path to use unix sep

Otherwise it would be hard to write uniform blacklist header list
across platforms, especially given that on Windows, some part of
path could be using '/' while others use '\'.

emilio/rust-bindgen,catdesk/rust-bindgen,catdesk/rust-bindgen,emilio/rust-bindgen,emilio/rust-bindgen,catdesk/rust-bindgen,rust-lang/rust-bindgen,rust-lang/rust-bindgen,jdub/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen,jdub/rust-bindgen,rust-lang/rust-bindgen,jdub/rust-bindgen,catdesk/rust-bindgen,jdub/rust-bindgen,emilio/rust-bindgen