netsniff-ng: Only use TPACKET_V3 if HAVE_TPACKET3 is defined

TPACKET_V3 is not defined if tpacket v3 is not available, thus make its
use conditional on HAVE_TPACKET3. Wrap the check for TPACKET_V3 in
ring_rx in an inline function which always returns false if
HAVE_TPACKET3 is not defined.

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