Added explicit DEFGENERIC form for (SETF SHEET-DIRECT-MIRROR).

The generic function (SETF SHEET-DIRECT-MIRROR) is not part of the
CLIM II spec, so there is no DEFGENERIC form for it in decls.lisp, and
also none in sheets.lisp.  Since some Common Lisp compilers emit
warnings when there is no explicit DEFGENERIC form, and to get a clean
build, I added such a form to sheets.lisp