Update MixpanelDestination.swift (#9)

Making the enable European endpoint in the destination setting an optional one so customers who have had this destination enabled for a while are able to migrate from iOS to Swift without any issues.
Currently, this causes the plugin to fail as the CDN file does not contain the enableEuropeanUnionEndpoint setting

Sources/SegmentMixpanel/MixpanelDestination.swift

@@ -327,7 +327,7 @@ extension MixpanelDestination: VersionedPlugin {
 
 private struct MixpanelSettings: Codable {
     let token: String
-    let enableEuropeanUnionEndpoint: Bool
+    let enableEuropeanUnionEndpoint: Bool?
     let consolidatedPageCalls: Bool
     let trackAllPages: Bool
     let trackNamedPages: Bool