selftests: txtimestamp: fix compilation issue

PACKET_TX_TIMESTAMP is defined in if_packet.h but it is not included in
test. Include it instead of <netpacket/packet.h> otherwise the error of
redefinition arrives.
Also fix the compiler warning about ambiguous control flow by adding
explicit braces.

Fixes: 8fe2f761 ("net-timestamp: expand documentation")
Suggested-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Signed-off-by: Vadim Fedorenko <vfedorenko@novek.ru>
Acked-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/1612461034-24524-1-git-send-email-vfedorenko@novek.ru


Signed-off-by: Jakub Kicinski <kuba@kernel.org>