package/iptraf-ng: really add _GNU_SOURCE in CFLAGS

When iptraf-ng is build with musl, it needs _GNU_SOURCE in CFLAGS to define
the content of "struct tcphdr".

iptraf-ng.mk try to add _GNU_SOURCE in CFLAGS but it's not taken into account.

Add it using IPTRAF_NG_CONF_ENV instead of IPTRAF_NG_MAKE_ENV.

Fixes:
http://autobuild.buildroot.net/results/a1b/a1b18f2e3d075d349c19536a7c5553f24b75a323



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>