Added NetworkBSD build flag to deal with Network.BSD moving to a new package.

Like with the network-uri split, cabal will automatically turn off the flag
when building with an old network.

I have not tested building with the new network-3.0.0.0 yet; several
other dependencies including aws are still pinned on network-2.*

stack.yaml

@@ -11,6 +11,7 @@ flags:
     dbus: false
     debuglocks: false
     benchmark: false
+    networkbsd: false
 packages:
 - '.'
 extra-deps: