soc: qcom: smem_state: Fix include for ERR_PTR()

The correct include file for getting errno constants and ERR_PTR() is
linux/err.h, rather than linux/errno.h, so fix the include.

Fixes: e8b123e6 ("soc: qcom: smem_state: Add stubs for disabled smem_state")
Acked-by: Andy Gross <andy.gross@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>