c++11 build warning on AArch32.DebianStretch.gcc63.SoftDocker

The recent change is to build with `-std=c++11` (#1820, #2020) which I believe is what triggered these warnings?

http://dynamorio.org/CDash/buildSummary.php?buildid=29765

```
/nightly/src/clients/drcachesim/tools/histogram.cpp: In member function 'virtual bool histogram_t::print_results()':
/nightly/src/clients/drcachesim/tools/histogram.cpp:91:56: note: parameter passing for argument of type 'std::__detail::_Node_iterator, false, false>' will change in GCC 7.1
top.begin(), top.end(), cmp);
^
```

Hopefully someone with access to this compiler is able to look more closely and try to find a fix?