textproc/gmetadom: Fix build with Clang 6

./../include/GdomeSmartDOMTraits.hh:101:25: error: non-constant-expression cannot be narrowed from type 'std::1::char_traits<unsigned short>::int_type' (aka 'unsigned long') to 'std::1::char_traits<unsigned short>::char_type' (aka 'unsigned short') in initializer list [-Wc++11-narrowing]

char_type __r = { __c };
                  ^~~

http://beefy8.nyi.freebsd.org/data/head-armv6-default/p475340_s336728/logs/gmetadom-0.2.6_4.log