Add extra env variable to set clang include paths

[kkettinger]

Unnecessary, as setting the CPATH environment variable does the same thing.

[molysgaard]

I've found this snippet to help a lot getting crates that use bindgen to compile correctly under bitbake/yocto.

BINDGEN_EXTRA_CLANG_ARGS = "--sysroot=${STAGING_DIR_TARGET}"
export BINDGEN_EXTRA_CLANG_ARGS