[Process] Fix linux arm64 single step compilation failure

This was updated in r356703 to use llvm::sys::RetryAfterSignal, which
comes from llvm/Support/Errno.h. The header wasn't added, so it fails if
you compile for arm64/aarch64.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@358530 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed