rfb: Silence nonliteral format string warning in Logger

Clang complains loudly about this with Wformat=2, so add
a __printf_attr to help out.

Fixes:

/home/shade/dev/tigervnc/common/rfb/Logger.cxx:48:35: error: format string is not a string literal [-Werror,-Wformat-nonliteral]
  vsnprintf(buf1, sizeof(buf1)-1, format, ap);
                                  ^~~~~~
1 error generated.
2 files changed