[CMake] Fix standalone build for debugserver on macOS

Summary:
Quick-fix to avoid CMake config issue:
```
CMake Error at /path/to/lldb/cmake/modules/AddLLDB.cmake:116 (add_dependencies):
  Cannot add target-level dependencies to non-existent target "lldb-suite".
```

Reviewers: xiaobai, beanz

Subscribers: mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D55032

llvm-svn: 347869