drivers: ethernet: native: fix compile error when glibc >= 2.20

Starting with glibc 2.20, there is warning when _BSD_SOURCE is defined
but not _DEFAULT_SOURCE (in /usr/include/features.h around line 184).
Sanitycheck turns this warning into error. So define _DEFAULT_SOURCE
at build time for native.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>