How to load Java based SecurityConfig in XML based Spring config file?

I already have a XML based Spring application. I have been asked to integrate Spring Security into it. I want to use Java based config. I have already initialized it as per the instructions here: Java Config but how and where do I load the SecurityConfig class.

My web.xml is:

<context-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>/WEB-INF/site.xml</param-value>
    </context-param>

    <listener>
        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
    </listener>

My SecurityWebApplicationInitializer is:

public class SecurityWebApplicationInitializer extends
AbstractSecurityWebApplicationInitializer {

}

My SecurityConfig is:

@EnableWebSecurity
public class SecurityConfig extends WebSecurityConfigurerAdapter {

    @Override
    protected void configure(HttpSecurity http) throws Exception {
        http.authorizeRequests().anyRequest().permitAll();
    }
}