main: Annotate format string args on new *_utf8 functions

The new charset translation functions need to use caller-supplied format
strings; this change corrects the following warnings:

      CC       openconnect-main.o
    main.c:422:22: warning: format string is not a string literal
          [-Wformat-nonliteral]
                    return vfprintf(f, fmt, args);
                                       ^~~
    main.c:424:29: warning: format string is not a string literal
          [-Wformat-nonliteral]
            ret = vasprintf(&utf8_str, fmt, args);
                                       ^~~

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>