Use installed header

Currently, openvino-rs requires a Git submodule, [upstream](https://github.com/intel/openvino-rs/tree/main/crates/openvino-sys) in order to retrieve the OpenVINO headers. When building openvino-rs using an existing installation (i.e., `OPENVINO_INSTALL_DIR`), we should be able to locate the C API header file directly (i.e., `${OPENVINO_INSTALL_DIR}/inference_engine/include/c_api/ie_c_api.h`). In these cases, we could attempt to use this header file to build openvino-sys.

Start in the linked crates/openvino-sys upstream area and review how the existing OpenVINO headers are retrieved. Use OPENVINO_INSTALL_DIR to locate inference_engine/include/c_api/ie_c_api.h, and consider the work complete when an installed OpenVINO can provide this header without requiring the Git submodule.

Written by the indexing model from the issue text.