Bug#30087 Set query_cache_size, if the value is too small, get a unclear warning

This bugs clarifies a warning message issued when the query cache data
size becomes smaller than the minium allowed size.

sql/sql_cache.h

@@ -245,6 +245,8 @@ private:
 
   void free_query_internal(Query_cache_block *point);
 
+  ulong get_minimal_size_limit();
+
 protected:
   /*
     The following mutex is locked when searching or changing global