mdadm: fix 07revert-inplace ptest

mdadm: fix 07revert-inplace ptest

Testcase 07revert-inplace fails if strace is not installed:
...
strace -o /tmp/str ./mdadm -A /dev/md0 --update=revert-reshape /dev/<...>
tests/07revert-inplace: line 40: strace: command not found

Add strace to mdadm-ptest RDEPENDS to make sure the testcase passes even with
a core-image-minimal build.

Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 7d9386663ac52ab69812867a0823c6055aedbc18)
Signed-off-by: Steve Sakoman <steve@sakoman.com>