fix build with --enable-terminal

Broke with 086891e5c119abb9854237fc32e736fe2d67234c
man: add systemd-sysv-generator(8)

In principle SysV stuff is only for compatibility, but we are stuck
with it for the forseeable future, so documentation might as well
be provided.

https://bugs.debian.org/771172
sysv-generator: advertise the man page