mail/spamprobe: Fix build with Clang/libc++

In file included from FrequencyDBImpl_cache.cc:34:
In file included from ./FrequencyDBImpl_cache.h:34:
In file included from ../../src/includes/LRUCache.h:34:
/usr/include/c++/v1/map:476:17: error: no matching function for call to object of type 'const LRUCache<std::__1::basic_string<char>, Ref<FrequencyDBImpl_cache::CacheEntry> >::LRUCacheNodeKeyCompare'

{return static_cast<const _Compare&>(*this)(__x.__cc.first, __y);}
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~