Enable -Wmismatched-tags in tao/PACKAGE

Summary:
This diff enables the titular warning flag for the directory in question. Further details are in [this workplace post](https://fb.workplace.com/permalink.php?story_fbid=pfbid02XaWNiCVk69r1ghfvDVpujB8Hr9Y61uDvNakxiZFa2jwiPHscVdEQwCBHrmWZSyMRl&id=100051201402394).

This is a low-risk diff. There are **no run-time effects** and the diff has already been observed to compile locally. **If the code compiles, it works; test errors are spurious.**

If the diff does not pass, it will be closed automatically.

Reviewed By: dmm-fb

Differential Revision: D53546469

fbshipit-source-id: d79d896579f92c98980fac60b6772f4dba9ce25e

cachelib/compact_cache/CCacheVariableLruBucket.h

@@ -220,7 +220,7 @@ struct VariableLruBucket {
     Bucket* bucket_;
     int pos_;
 
-    friend class VariableLruBucket<CompactCacheDescriptor>;
+    friend struct VariableLruBucket<CompactCacheDescriptor>;
   };
 
   /** Type of the callback to be called when an entry is evicted. */