clang: fix struct/class declaration/definition mismatches

clang does not like a defined class to be declared as a struct.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

src/bees.h

@@ -165,7 +165,7 @@ public:
 	T at(string idx) const;
 
 friend ostream& operator<< <>(ostream &os, const BeesStatTmpl<T> &bs);
-friend class BeesStats;
+friend struct BeesStats;
 };
 
 using BeesRates = BeesStatTmpl<double>;