netsniff-ng: Properly wrap usage of all tpacket v3 structs

Mike Reeves reports the following compilation error if tpacket v3 is not
available:

>  CC   ring_rx.c
> ring_rx.c: In function 'alloc_rx_ring_frames':
> ring_rx.c:143: error: 'struct ring' has no member named 'layout3'
> ring_rx.c:144: error: 'struct ring' has no member named 'layout3'
> ring_rx.c: In function 'sock_rx_net_stats':
> ring_rx.c:172: error: field 'k3' has incomplete type
> make: *** [netsniff-ng/ring_rx.o] Error 1

The layout3 member of struct ring is only available for HAVE_TPACKET3.
Thus, wrap all access to it into inline functions defined depending on
wheter HAVE_TPACKET3 is defined.

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