dt-utils: fix hash

Hash has been reported as wrong by autobuilder since August 16th
Perhaps, it was wrong since bump to v2018.05.0 on August 9th

Fixes:
 - http://autobuild.buildroot.org/results/c6c8ede30623cc20011916cc95dbdae34dcc0344



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Marcin Niestroj <m.niestroj@grinn-global.com>
Tested-by: Marcin Niestroj <m.niestroj@grinn-global.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>