games/widelands: unbreak with ICU 61

In file included from src/graphic/font_handler.cc:36:
src/graphic/text/bidi.h:43:36: error: unknown type name 'UnicodeString'; did you mean 'icu::UnicodeString'?
std::string icustring2string(const UnicodeString& convertme);

^~~~~~~~~~~~~
icu::UnicodeString

/usr/local/include/unicode/unistr.h:286:20: note: 'icu::UnicodeString' declared here
class U_COMMON_API UnicodeString : public Replaceable

^

https://ssl.icu-project.org/trac/changeset/40705

PR: 227042
Reported by: antoine (via exp-run)
Obtained from: upstream