libevent: switch to patches-2.0 branch to get the appropriate automake fix

libevent 2.0.21 is not compatible with automake 2.13. libevent 2.0.22
will be but is not released.

diff --git a/libevent b/libevent
index 64177777165d9684bafbfa946abd126f7ebff11f..e826f19e05b65ffea1a1699160cee7516962c939 160000 (submodule)
--- a/libevent
+++ b/libevent
@@ -1 +1 @@
-Subproject commit 64177777165d9684bafbfa946abd126f7ebff11f
+Subproject commit e826f19e05b65ffea1a1699160cee7516962c939