selftests/bpf: add missing header include for htons

[ Upstream commit bc9b3fb8 ]

Including the network_helpers.h header in tests can lead to the following
build error:

./network_helpers.h: In function ‘csum_tcpudp_magic’:
./network_helpers.h:116:14: error: implicit declaration of function \
  ‘htons’ [-Werror=implicit-function-declaration]
  116 |         s += htons(proto + len);

The error is avoided in many cases thanks to some other headers included
earlier and bringing in arpa/inet.h (ie: test_progs.h).

Make sure that test_progs build success does not depend on header ordering
by adding the missing header include in network_helpers.h

Fixes: f6642de0 ("selftests/bpf: Add csum helpers")
Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
Link: https://lore.kernel.org/r/20241008-network_helpers_fix-v1-1-2c2ae03df7ef@bootlin.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>