Fix compile error on iOS

[torufuruya]

/node_modules/mixpanel-react-native/ios/MixpanelReactNative.swift:302:49: error: type 'MixpanelTypeHandler' has no member 'ToMixpane
lType'
            mpGroups[key] = MixpanelTypeHandler.ToMixpanelType(value)
                            ~~~~~~~~~~~~~~~~~~~ ^~~~~~~~~~~~~~
/node_modules/mixpanel-react-native/ios/MixpanelReactNative.swift:313:77: error: type 'MixpanelTypeHandler' has no member 'ToMixpane
lType'
        instance?.setGroup(groupKey: groupKey, groupID: MixpanelTypeHandler.ToMixpanelType(groupID)!)
                                                        ~~~~~~~~~~~~~~~~~~~ ^~~~~~~~~~~~~~
/node_modules/mixpanel-react-native/ios/MixpanelReactNative.swift:322:77: error: type 'MixpanelTypeHandler' has no member 'ToMixpane
lType'
        instance?.addGroup(groupKey: groupKey, groupID: MixpanelTypeHandler.ToMixpanelType(groupID)!)
                                                        ~~~~~~~~~~~~~~~~~~~ ^~~~~~~~~~~~~~
/node_modules/mixpanel-react-native/ios/MixpanelReactNative.swift:331:80: error: type 'MixpanelTypeHandler' has no member 'ToMixpane
lType'
        instance?.removeGroup(groupKey: groupKey, groupID: MixpanelTypeHandler.ToMixpanelType(groupID)!)
                                                           ~~~~~~~~~~~~~~~~~~~ ^~~~~~~~~~~~~~
/node_modules/mixpanel-react-native/ios/MixpanelReactNative.swift:340:61: error: type 'MixpanelTypeHandler' has no member 'ToMixpane
lType'
        guard let mixpanelTypeGroupID = MixpanelTypeHandler.ToMixpanelType(groupID) else {
                                        ~~~~~~~~~~~~~~~~~~~ ^~~~~~~~~~~~~~
/node_modules/mixpanel-react-native/ios/MixpanelReactNative.swift:394:63: error: type 'MixpanelTypeHandler' has no member 'ToMixpane
lType'
            guard let mixpanelTypeValue = MixpanelTypeHandler.ToMixpanelType(value) else {
                                          ~~~~~~~~~~~~~~~~~~~ ^~~~~~~~~~~~~~
/node_modules/mixpanel-react-native/ios/MixpanelReactNative.swift:410:79: error: type 'MixpanelTypeHandler' has no member 'ToMixpane
lType'
            group.union(key: name, values: values.map() { MixpanelTypeHandler.ToMixpanelType($0)! })

It would have caused by this change

[zihejia]

Thank @torufuruya for providing this and sorry about the error.