libevent: update embedded copy to 2.0.22-stable

diff --git a/libevent b/libevent
index e826f19e05b65ffea1a1699160cee7516962c939..c51b159cff9f5e86696f5b9a4c6f517276056258 160000 (submodule)
--- a/libevent
+++ b/libevent
@@ -1 +1 @@
-Subproject commit e826f19e05b65ffea1a1699160cee7516962c939
+Subproject commit c51b159cff9f5e86696f5b9a4c6f517276056258