ptimer: express dependencies with Kconfig

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Yang Zhong <yang.zhong@intel.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20190123065618.3520-39-yang.zhong@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>