[lldb] Allows customizing libxml2 for darwin

Summary:
This changes allows to disable or use customized libxml2 for lldb.

1. Removes redundant include_directories. The one in LLDBConfig.cmake should be enough.

2. Link to ${LIBXML2_LIBRARIES} if xml2 is enabled.

Subscribers: mgorny, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D80257