[stream_executor:host] Add missing externs to C API header
[stream_executor:host] Add missing externs to C API header

FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/openxla/xla/pull/10759 from Intel-tensorflow:akhil/gemv 4534fbd7e96a6e45ccc5a501b38b940aa8bd9d38
