Make sure wp_suspend_cache_addition() exists

When using wp-redis along with Batcache, a fatal error is triggered because Batcache loads prior to wp_suspend_cache_addition() being defined. 

@tollmanz ran into a similar issue with his memcached backand + Batcache https://github.com/tollmanz/wordpress-pecl-memcached-object-cache/pull/45
Merge pull request #2 from ryanduff/patch-1

Make sure wp_suspend_cache_addition() exists