devel/uatraits: Fix build with Clang 6

../include/uatraits/details/regex_utils.hpp:67:24: error: non-constant-expression cannot be narrowed from type 'decltype(x.base() - y.base())' (aka 'int') to 'std::size_t' (aka 'unsigned int') in initializer list [-Wc++11-narrowing]

regex_data data = { i - begin, pos - begin, scan_integer<std::size_t>(i + 1, pos) };
                    ^~~~~~~~~

http://beefy11.nyi.freebsd.org/data/head-i386-default/p477696_s338122/logs/errors/uatraits-20130430_11.log

Approved by: demon (maintainer, implicit)