selftests: kvm: add tsc_scaling_sync to .gitignore

The tsc_scaling_sync's binary should be present in the .gitignore
file for the git to ignore it.

Signed-off-by: Like Xu <likexu@tencent.com>
Message-Id: <20220406063715.55625-3-likexu@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>