Fix building without S3 support

http://hydra.nixos.org/build/49031196/nixlog/2/raw

src/libstore/download.cc

@@ -489,7 +489,7 @@ struct CurlDownloader : public Downloader
         /* Ugly hack to support s3:// URIs. */
         if (hasPrefix(request.uri, "s3://")) {
             // FIXME: do this on a worker thread
-            sync2async<DownloadResult>(success, failure, [&]() {
+            sync2async<DownloadResult>(success, failure, [&]() -> DownloadResult {
 #ifdef ENABLE_S3
                 S3Helper s3Helper;
                 auto slash = request.uri.find('/', 5);

src/libstore/legacy-ssh-store.cc

@@ -225,7 +225,7 @@ struct LegacySSHStore : public Store
     void collectGarbage(const GCOptions & options, GCResults & results) override
     { unsupported(); }
 
-    ref<FSAccessor> getFSAccessor()
+    ref<FSAccessor> getFSAccessor() override
     { unsupported(); }
 
     void addSignatures(const Path & storePath, const StringSet & sigs) override