add fedora-26 to the SANITY_TESTED_DISTROS

tested the quickstart guide on Fedora 26 and confirmed that it
can build a core-image-sato and run the image via runqemu.

(From meta-yocto rev: 4e719fb77d035123d0e2ff84d10cb68c7cc8eccd)

Signed-off-by: Benjamin Esquivel <code@esquibits.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

@@ -72,6 +72,7 @@ SANITY_TESTED_DISTROS ?= " \
             ubuntu-17.04 \n \
             fedora-24 \n \
             fedora-25 \n \
+            fedora-26 \n \
             centos-7 \n \
             debian-8 \n \
             opensuse-42.1 \n \