Build: Always use --no-discard-stderr when running help2man.

We need it in the case where there's not a man page include file for a
tool that's been converted to using glib. In this case, crm_standby
calls through to crm_attribute when given --version. The version
information is written to stderr (due to pcmk__cli_help), but there's no
man page include file. help2man sees no version output where it's
expected and errors.