Fix "performance-for-range-copy" error.

Not sure why this only shows up with -std=c++17, but it's clearly
correct:

  art/tools/hiddenapi/hiddenapi.cc:1063:45: error: the loop variable's type is not a reference type; this creates a copy in each iteration; consider making this a reference [performance-for-range-copy,-warnings-as-errors]

Bug: http://b/111067277
Test: builds
Change-Id: I3796d275b142db3de40e7a855a8130ecea6212ae