[NFC] Add missing include to LaneBitmask.h to fix modules build
http://green.lab.llvm.org/green/view/LLDB/job/lldb-cmake/ is building LLVM + Clang + LLDB with modules. IIRC the bot isn't sending mails as Green Dragon's build nodes are not that reliable.
FWIW, I believe there is no requirement for LLVM devs to watch out for module failures. Me + others who are using it for bots/personal use will usually fix the bugs when they appear (in part because the modules builds right now are working best on macOS and some modified Linux systems and we can't expect people to have access to that). So there is no need to watch the Green Dragon bot yourself.