firmware: qcom: scm: fix compilation error when disabled

This fixes the case when CONFIG_QCOM_SCM is not enabled, and linux/errno.h
has not been included previously.

Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>