hikey: Fixes compiler error for Debian builds

Related to: https://github.com/OP-TEE/optee_os/issues/832



Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Tested-by: Joakim Bech <joakim.bech@linaro.org> (HiKey Debian)
Suggested-by: Machiry Aravind Kumar <makrvcs@gmail.com>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>