ltp: add RDEPENDS on findutils

With busybox find some of the test script fails, so depend on GNU find.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit 2d1b1987)
Signed-off-by: Steve Sakoman <steve@sakoman.com>