undefined symbol: ap_cache_cacheable_hdrs_out

After enabling mod_mem_cache, I encountered these errors when restarting apache:

* Restarting web server apache2
 * We failed to correctly shutdown apache, so we're now killing all running apache processes. This is almost certainly suboptimal, so please make sure your system is working as you'd expect now!
 ... waiting apache2: Syntax error on line 204 of /etc/apache2/apache2.conf: Syntax error on line 2 of /etc/apache2/mods-enabled/mem_cache.load: Cannot load /usr/lib/apache2/modules/mod_mem_cache.so into server: /usr/lib/apache2/modules/mod_mem_cache.so: undefined symbol: ap_cache_cacheable_hdrs_out
   ...fail!

Just put:

LoadModule cache_module /usr/lib/apache2/modules/mod_cache.so

before:

# Include module configuration:

in:

/etc/apache2/apache2.conf

And it's fixed!