gerrit-config: Add robots.txt also for gitiles

Change-Id: I38b246394b6781666590f2f7782fdcb560239fce
diff --git a/nginx/git.leafos.org b/nginx/git.leafos.org
index 83b0308..e32f56d 100644
--- a/nginx/git.leafos.org
+++ b/nginx/git.leafos.org
@@ -63,6 +63,9 @@
 	location /favicon.ico {
 		proxy_pass http://localhost:8080;
 	}
+	location /robots.txt {
+		proxy_pass http://localhost:8080/robots.txt;
+	}
 
 	# git clone
 	location ~ ^.*/(HEAD|info/refs|objects/info/.*|git-(upload|receive)-pack)$ {