libevent: upgrade to 2.1.11-stable

diff --git a/libevent b/libevent
index c51b159cff9f5e86696f5b9a4c6f517276056258..4c908dde58ef780eeefcc9df4db3063ca62ea862 160000 (submodule)
--- a/libevent
+++ b/libevent
@@ -1 +1 @@
-Subproject commit c51b159cff9f5e86696f5b9a4c6f517276056258
+Subproject commit 4c908dde58ef780eeefcc9df4db3063ca62ea862