subsys shell devmem_service: Fix includes

* The include for the host libC should be guarded with
  using the host libC instead of the POSIX arch, as this
  supports other C libraries.
* This code uses getopt, which is an extension to the
  C library. Let's explicity select one of the extensions
  which includes it instead of relaying on somebody having
  set it for this file somewhere else.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>