Fix getS3Stats()

src/libstore/s3-binary-cache-store.cc

@@ -99,7 +99,7 @@ struct S3BinaryCacheStoreImpl : public S3BinaryCacheStore
         }
     }
 
-    const Stats & getS3Stats()
+    const Stats & getS3Stats() override
     {
         return stats;
     }

src/libstore/s3-binary-cache-store.hh

@@ -27,7 +27,7 @@ public:
         std::atomic<uint64_t> head{0};
     };
 
-    const Stats & getS3Stats();
+    virtual const Stats & getS3Stats() = 0;
 };
 
 }