package/iptraf-ng: Needs -D_GNU_SOURCE to fix musl build

package/iptraf-ng: Needs -D_GNU_SOURCE to fix musl build

Compiling iptraf-ng using musl is broken:

src/ipfrag.c:193:52: error: 'struct tcphdr' has no member named 'source'
    ftmp->s_port = ntohs(((struct tcphdr *) tpacket)->source);

Musl guards these fields in the netinet/tcp.h header file with
_GNU_SOURCE, for details see https://dev.openwrt.org/ticket/19889

Patch ported from
http://git.alpinelinux.org/cgit/aports/tree/main/iptraf-ng/APKBUILD#n28

Fixes
http://autobuild.buildroot.net/results/4ce/4cedd4dbaafca76e839f45cc0a4bb8c2c181920d/
http://autobuild.buildroot.net/results/72b/72b019ce5bb52b8b87d92cce675708f7b9f8a96e/
http://autobuild.buildroot.net/results/f67/f67f1c2a627a6d535dd5c00e2ee131b5c5a99a0a/

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Tested-by: Jörg Krause <joerg.krause@embedded.rocks>
[Thomas: one assignment only for IPTRAF_NG_CONF_ENV.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>