Fix lldb build on Mac.

Summary:
https://github.com/llvm-mirror/lldb/commit/e80f43fd78fa0fbc04d2d59b5713acb5d06c8308
greatly improved an API, but missed one more occurence of legacy usage.

This leads to:
  if (extractor.GetHexBytes(&payload_bytes[0], payload_bytes.size(), '\xdd') != payload_bytes.size())
                                ~~~~~~~~~~~~~~~~~~~~~                                          ^~~~~~
  /lldb/include/lldb/Utility/StringExtractor.h:151:5: note: 'GetHexBytes' declared here

Reviewers: zturner

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D24064
Author: Taras Tsugrii <ttsugrii@fb.com>

llvm-svn: 280202