zdtm: fix struct tpacket_req3 redeclaration

Linux has it since v3.2

cc -g -O2 -Wall -Werror -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=0 -I../../lib   packet_sock.c ../../lib/libzdtmtst.a   -o packet_sock
packet_sock.c:70:8: error: redefinition of ‘struct tpacket_req3’
In file included from packet_sock.c:23:0:
/usr/include/linux/if_packet.h:245:8: note: originally defined here

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>