Xcode 10: Adjust Build Phases order
Xcode 10: Adjust Build Phases order

`Headers` should be placed before `Sources` to match with the Xcode 10's project template.

AnthonyMDev/Nimble,Quick/Nimble,phatblat/Nimble,twobitlabs/Nimble,Quick/Nimble,AnthonyMDev/Nimble,AnthonyMDev/Nimble,twobitlabs/Nimble,Quick/Nimble,phatblat/Nimble,twobitlabs/Nimble,phatblat/Nimble