added missing android/build git ignore
added missing android/build git ignore

devshackio/react-native-firestack,sraka1/react-native-firestack,devshackio/react-native-firestack,devshackio/react-native-firestack,sraka1/react-native-firestack,devshackio/react-native-firestack,sraka1/react-native-firestack,sraka1/react-native-firestack