Include what you use in LanaiMCCodeEmitter.cpp

Include what you use in LanaiMCCodeEmitter.cpp

LanaiMCCodeEmitter.cpp was not using any APIs from Lanai.h, and was only
including it for transitive dependencies.  Doing so is problematic from
include-what-you-use perspective, but it is also a layering issue (it
creates a dependency cycle between the primary Lanai target library and
the MCTargetDesc library).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@362394 91177308-0d34-0410-b5e6-96231b3b80d8