Translations of QKeyChain depend on the target platform

QKeyChain has translatable strings in keychain_apple.mm, keychain_unix.cpp, keychain_win.cpp, and we currently ever only get one set of translations to lupdate. Investigate how to best solve this.
(Passing the additional files to lupdate? But can that parse these correctly without the correct includes? Moving the trs out of the platform specific files (and fixing upstream)?)