From Gisle Vanem: use the right #define when checking whether libpcap

From Gisle Vanem: use the right #define when checking whether libpcap
has "pcap_dump_flush()".

The WinDump makefiles define HAVE_PCAP_FINDALLDEVS, and will be changed
to define HAVE_PCAP_DUMP_FLUSH, so use thos, rather than Win32, to check
whether WinPcap has "pcap_findalldevs()" or "pcap_dump_flush()", so
people can tweak the appropriate makefile if building WinDump with an
older version of WinPcap lacking one or both of those routines.