Check that C header is present

[abrown]

Though not necessary, it is nice to include the C header along with the openvino-sys crate in case someone in the future needs to compare the bindgen result with the original C code. In v0.5.0, the header was not included (likely because the upstream directory was not checked out): https://sourcegraph.com/crates/openvino-sys/-/compare/v0.4.2...v0.5.0. We could add a test to check that the file is present to avoid this in the future.