llvm-readobj: Add missed source file

In some configurations (e.g. powerpc64) the llvm-readobj tool also needs
contrib/llvm-project/llvm/BinaryFormat/MsgPackWriter.cpp, so add it to
libllvm.

Reported by: Shawn Webb <shawn.webb@hardenedbsd.org>
Fixes: 1b85b68da0b2