When building the current version of notmuch on debian testing/unstable,
i see the following warning:

```
EMACS emacs/notmuch-crypto.elc

In end of data:
emacs/notmuch-crypto.el:266:1:Warning: the function
    ‘notmuch-show-get-message-id’ is not known to be defined.
```

No part of the test suite fails so i assume it's not a problem, but
having spurious warnings makes it harder to notice real warnings in the
future.

I don't know what the warning means exactly, but maybe someone who is
better with elisp than i am could take a look at it and try to resolve
it?

thanks,

        --dkg