torture: Add "ERROR" diagnostic for testing kernel-build output

Some recent kernel-build failures have featured "ERROR", so this commit
adds it to the list checked by kvm-build.sh.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.upadhyay@kernel.org>