Use installed header

[abrown]

Currently, openvino-rs requires a Git submodule, upstream 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.

[chemicstry]

I think it would make sense to remove openvino submodule from the repostiroy completely and only clone when needed in build.rs like other libs do. Otherwise using openvino-rs as a git dependency recursively clones all ~20 repositories, which totals to 2.5 GB of downloads. openvino-rs as git dependency is quite useful for testing new features/patches.

[abrown]

@chemicstry, that's a good idea; I would be happy to merge a PR along those lines. Originally I had the hope of keeping the Build from OpenVINO sources mechanism alive but at some point recently that became significantly more difficult and the from-source build option was removed (#39). Now that that is gone, it makes it much easier to only retrieve the necessary header files when generating the bindings, which is probably only necessary when the cargo xtask codegen task is run?