gnome-themes: fix path to icon-name-mapping

* it was pointing to
  UTILS_PATH=/OE/jansa-test/shr-core/tmp-eglibc/sysroots/x86_64-linux/usr/lib/gnome-themes
  after libexecdir change in oe-core

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>