[lldb] [cmake] Fix typo in unittest directory path

Fix a typo in a11cd0d9 that resulted
in additional "}" in unittest directory path, e.g.:

    CMake Error at cmake/modules/LLDBStandalone.cmake:104 (add_subdirectory):
      add_subdirectory given source
      "/var/tmp/portage/dev-util/lldb-16.0.0_pre20221111/work/lldb/../third-party}/utils/unittest"
      which is not an existing directory.
    Call Stack (most recent call first):
      CMakeLists.txt:29 (include)