Fix up r312105

• Only #include tcpd.h when LIBWRAP is true to avoid header include errors
• Only define whichaf when LIBWRAP is true to avoid -Wunused warning and to avoid issues with structs being defined that should only be defined when tcpd.h is included.

MFC after: 2 weeks
X-MFC with: r312105
Pointyhat to: ngie
Reported by: gcc tinderbox
Sponsored by: Dell EMC Isilon