python: Don't install nis_failed.so if no yp headers

Because python won't even try to build it.
You only see nis_failed.so if there is an error building it, when
it wanted to build it, and that should be fixed accordingly.

In a nutshell, if the yp headers are installed, python will build
the nis module, otherwise it won't.

On netbsd systems at least, if you have the yp headers installed and
subsequently re-install over the top with MKYP=no you get into the state
where the headers are installed, but the functions are no longer in libc.
This is an error with *your* system - either rebuild with MKYP=yes OR
remove the yp headers from include/rpc and include/rpcsvc.

Follow on fix for PR pkg/53673.

(roy)