Yes, because if you build Qt 5 with ICU to enable WebKit QtCore will also take advantage of it.
As sierdzio has already mentioned you can disable ICU using the -no-icu flag so that QtCore should no longer depend on it. But you can't build WebKit without ICU.
You might be able to build Qt 5 without ICU and WebKit with ICU using different configurations, but I haven't tried that yet.