multimedia/aegisub: unbreak with boost 1.68

src/search_replace_engine.cpp:315:14: error: call to

'distance' is ambiguous
                          count += distance(
                                   ^~~~~~~~

/usr/include/c++/v1/iterator:511:1: note: candidate function [with _InputIter =

boost::u32regex_iterator<std::__1::__wrap_iter<const char *> >]

distance(_InputIter first, _InputIter last)
^
/usr/local/include/boost/iterator/distance.hpp:49:9: note: candidate function [with SinglePassIterator =

boost::u32regex_iterator<std::__1::__wrap_iter<const char *> >]
  distance(SinglePassIterator first, SinglePassIterator last)
  ^

PR: 229569