fix(ios): add <string> and <memory> includes in RNDatePicker.mm#941

Sosotess93 wants to merge 2 commits intohenninghall:masterhenninghall/react-native-date-picker:masterfrom

Sosotess93:fix/ios-cpp-includes-rndatepickerSosotess93/react-native-date-picker:fix/ios-cpp-includes-rndatepickerCopy head branch name to clipboard