rm_work.bbclass: Fix for files starting with -

This makes it possible to name files starting with a hyphen in the work
directory. Without this change rm will fail due to an unexpected option
being passed.

Signed-off-by: William A. Kennington III <wak@google.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 5f1a63e0)
Signed-off-by: Steve Sakoman <steve@sakoman.com>