net/yami4: unbreak with GCC < 6

posix/io_error_handler.cpp: In function 'void yami::details::handle_io_error_with_code(const char*, int, void (*)(void*, int, const char*), void*)':
posix/io_error_handler.cpp:45: error: 'snprintf' is not a member of 'std'

Reported by: pkg-fallout (mips64)