package/aircrack-ng: fix musl build

Backport an upstream patch [1] and remove sys/cdefs.h from
ethernet.h as suggested by the musl wiki [2].

Fixes:
http://autobuild.buildroot.net/results/9721496871d104b4cfb68ce133b5f48612cb7e50/

[1] https://github.com/aircrack-ng/aircrack-ng/commit/42de9f800056601443ac12edbba7bd5802740db2
[2] http://wiki.musl-libc.org/wiki/FAQ



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