We had this problem too in one of our targets in the same project where other targets were building fine. We finally fixed it by adding:

#import <WebKit/WebKit.h>

To our per-compiled header file - ProjectName.pch

( thanks to https://github.com/cedarbdd/cedar/issues/397 for the clue)

It's a mystery why this solved it. We assume it was something to do with the order in which headers were being included for some mysterious reason best beknown to the Swift & Objective C compiler gurus at Apple. Would be good if Apple fixed it....