fix check-wrkref path

["Masao Uebayashi" <uebayasi%gmail.com@localhost>]

This fixes check-wrkref paths .

        http://www.NetBSD.org/~uebayasi/fix-check-wrkref-path.patch

_CHECK_WRKREF_FILELIST_CMD is a command which prints PLIST paths
converted to absolute path.  _CHECK_WRKREF_FILELIST_CMD can run
anywhere, so cd ${DESTDIR} before that is pointless.  To access files,
_CHECK_WRKREF_FILELIST_CMD's output needs to be s/^/${DESTDIR}/ when
destdir is used.

(I have no idea how long this has been broken. :)

Masao