net/mlx: fix compilation with glibc 2.20

net/mlx: fix compilation with glibc 2.20

Since _BSD_SOURCE was deprecated in favor of _DEFAULT_SOURCE in Glibc 2.19
and entirely removed in 2.20, various BSD ioctl macros are not exposed
anymore when _XOPEN_SOURCE is defined, and linux/if.h now conflicts with
net/if.h.

Add _DEFAULT_SOURCE and keep _BSD_SOURCE for compatibility with older
versions.

Suggested-by: Bruce Richardson <bruce.richardson@intel.com>
Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>