Swift 5 migration: Fix switch warnings

We now get a warning when switching over enums from system
frameworks even when we handle all public cases because
there can be future cases that aren't handled.

When such a future case is introduced, we'll get a warning.

Signed-off-by: Roopesh Chander <roop@roopc.net>

WireGuard/WireGuardNetworkExtension/DNSResolver.swift

@@ -117,6 +117,8 @@ extension Endpoint {
             hostname = "\(address)"
         case .ipv6(let address):
             hostname = "\(address)"
+        @unknown default:
+            fatalError()
         }
 
         var resultPointer = UnsafeMutablePointer<addrinfo>(OpaquePointer(bitPattern: 0))