ext: simplelink: Enable SimpleLink to use Zephyr __errno

The SimpleLink host driver comes with its own definition of
__errno, which conflicts with Zephyr's definition, but has
a mechanism to enable use of the porting OS's __errno variable.

This patch enables SimpleLink to use Zephyr's __errno
via the DPL porting layer.

Jira: ZEP-1958

Signed-off-by: Gil Pitney <gil.pitney@linaro.org>