`plan.pinv` undefined reference

Feel free to close this if this is expected/tolerated. I just happened across this error when trying to serialize an object that contained an FFTPlan, and the serializer (BSON) trips the `undefined reference` error. It'd be nice if it didn't error out, but I also understand it's a weird case.

```
using FFTW

plan = plan_fft(rand(20,20))
plan.pinv

ERROR: UndefRefError: access to undefined reference
```

Start with the Julia reproduction using plan_fft(rand(20,20)) and accessing plan.pinv, then serialize an object containing the FFTPlan with BSON. Trace the undefined reference and determine whether serialization should tolerate it; done means the reported serialization path no longer raises UndefRefError, with the expected behavior documented or tested.

Written by the indexing model from the issue text.