netsniff-ng: Move variable definition

Save one #ifdef block by moving the tpacket v3 only variable definition
to the block where it is actually used.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
ring_rx: Fix typo in header guard #endif

It's HAVE_TPACKET3, not HAVE_TPACKETV3.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
netsniff-ng: Fix tpacketv2-only capturing

We need to set up the RX ring depending on whether tpacket v3 is
available or not. Otherwise end up setting its structure up for tpacket
v3, even though only tpacket v2 is available. This should fix packet
capturing for tpacket v2 (i.e. corrupted frames in pcap).

Reported-by: Mike Reeves <luke@geekempire.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>