poky: add debian-9 to SANITY_TESTED_DISTROS

I have been working with OE on debian-9 for several months now without
issue. In addition, I tested a build + runqemu for core-image-sato
following the quickstart guide and had no issues.

(From meta-yocto rev: 2d8a572df7498ce8eb3a78f05384f0f7dd5ecb91)

Signed-off-by: Martin Kelly <mkelly@xevo.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta-poky/conf/distro/poky.conf

@@ -75,6 +75,7 @@ SANITY_TESTED_DISTROS ?= " \
             fedora-26 \n \
             centos-7 \n \
             debian-8 \n \
+            debian-9 \n \
             opensuse-42.1 \n \
             opensuse-42.2 \n \
             "