Add cffi-grovel to :depends-on

[Ambrevar]

Else hdf5-cffi fails to load in a couple of cases.
See
https://gitlab.common-lisp.net/asdf/asdf/issues/23
for a discussion.
Thanks!

[khinsen]

Note: this issue arose when packaging hdf5-cffi for the Guix package manager, so it's not a purely theoretical concern.