drivers: ethernet: native: Fix compile issue in RedHat 7

Fix 2 compile issues in RedHat/CentOS 7 for the native
ethernet driver due to a oldish glibc, which does not include
by default the needed structures.
+
Separate the native ethernet driver into its own library,
and set NO_POSIX_CHEATS for it, in case the Zephyr POSIX
library would eventually add support for any of the host functions
used in this driver.

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>