dhcpdump: fix build with musl

muls exposes udphdr Linux member names only when _GNU_SOURCE is defined.

Fixes:
http://autobuild.buildroot.net/results/dc2/dc2902b758faa7bcfed44cca5fa73aeffdf284ae/
http://autobuild.buildroot.net/results/432/43214b5f7d9e5881fbeba7203ee7fcfc6386a492/
http://autobuild.buildroot.net/results/291/29100641731904b50d73af7cf2db08847fe1567a/



and more.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>