[Xcode] error build: 'FirebaseCore/FirebaseCore.h' file not found 에러 해결방법

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<FirebaseCore/FirebaseCore.h> file not found

이것을

#import "Firebase.h"

이것으로 변경한다

참고 : https://stackoverflow.com/questions/52077002/header-file-firebasecore-firebasecore-h-not-found-despite-importing-firebase