sandbox: eth-raw: fix building with musl library

The definition of struct udphdr in include netinet/udp.h in the
musl library differs from the definition in the glibc library.

To use the same definition with musl the symbol _GNU_SOURCE has
to be defined.

Reported-by: Milan P. Stanić <mps@arvanta.net>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Tested-by: Milan P. Stanić <mps@arvanta.net>
1 file changed