ltp: add iputils-ping6 to RDEPENDS

According to the LTP documents[1], it using ping6 from iputils which
support "-I" option to bind the interface[2].

Ref:
[1] https://github.com/linux-test-project/ltp/commit/6f97789c
[2] https://github.com/linux-test-project/ltp/commit/64b11656



Signed-off-by: Xiangyu Chen <xiangyu.chen@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 05107ec5)
Signed-off-by: Bin Lan <bin.lan.cn@windriver.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>