enable xmpp in stack.yaml

Since stack is being used to build the OSX autouild now, I want xmpp
enabled.

This means stack can't be used to build git-annex on windows, unless the
user edits this file and disables xmpp. Unfortunate that stack is so
unconfigurable, compared with cabal..

stack.yaml

@@ -11,7 +11,7 @@ flags:
     assistant: true
     pairing: true
     cryptonite: true
-    xmpp: false
+    xmpp: true
     network-uri: true
     s3: true
     dbus: false