Fix buildability in SBCL

From Christophe Rhodes: Effective slot definitions don't (in the MOP)
have readers or writers; those are attached to direct slots.  SBCL is
now enforcing this, and this change is backwards-compatible because
the default for the slots was NIL in any case.
Merge pull request #6 from quicklisp/patch-1

Fix buildability in SBCL