Added MixpanelWatchOS as a public header file.
Added MixpanelWatchOS as a public header file.

mixpanel/mixpanel-iphone,mixpanel/mixpanel-iphone,depop/mixpanel-iphone,carezone/mixpanel-iphone,depop/mixpanel-iphone,mixpanel/mixpanel-iphone,mixpanel/mixpanel-iphone,depop/mixpanel-iphone,carezone/mixpanel-iphone,depop/mixpanel-iphone,mixpanel/mixpanel-iphone