Properly define {u_}intN_t types when testing pcap/*.h headers.

Properly define {u_}intN_t types when testing pcap/*.h headers.

pcap/bluetooth.h and pcap/usb.h require those types, so we check for
those types before checking those headers, and we include
"tcpdump-stdinc.h" in the tests, so the appropriate headers to define
those types get pulled in.