Include <netdb.h> before <pcap-namedb.h>, as <netdb.h> is included to

Include <netdb.h> before <pcap-namedb.h>, as <netdb.h> is included to
define a structure used by <pcap-namedb.h> (only a pointer to the
structure is used in <pcap-namedb.h>, so code will compile no matter
which order you include them in, but it's a bit cleaner to include
<netdb.h> first).  Indicate why we're including <netdb.h>.