Correctly call all the parent contructors of the stores

Using virtual inheritance means that only the default constructors of
the parent classes will be called, which isn't what we want

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

@@ -31,7 +31,9 @@ public:
     HttpBinaryCacheStore(
         const Path & _cacheUri,
         const Params & params)
-        : BinaryCacheStore(params)
+        : StoreConfig(params)
+        , BinaryCacheStoreConfig(params)
+        , BinaryCacheStore(params)
         , HttpBinaryCacheStoreConfig(params)
         , cacheUri(_cacheUri)
     {