Fix build with dbus on FreeBSD.

pcap-dbus.c fails to compile if the dbus package is installed on
FreeBSD. It fails with:

./pcap-dbus.c: In function 'dbus_read':
./pcap-dbus.c:87: error: 'struct pcap_md' has no member named 'packets_read'
./pcap-dbus.c: In function 'dbus_stats':
./pcap-dbus.c:122: error: 'struct pcap_md' has no member named 'packets_read'

Fix this by moving packets_read out from the linux ifdef.