[2202] Unify class-struct

It should be the same according to the standard, but Clang seems not to
agree with that.

src/lib/util/threads/lock.h

@@ -125,7 +125,7 @@ public:
     bool locked() const;
 private:
     friend class Locker;
-    struct Impl;
+    class Impl;
     Impl* impl_;
     void lock();
     void unlock();