clang workaround of vsnprintf

clang workaround of vsnprintf

With clang in macos:

/Users/runner/work/nnstreamer/nnstreamer/gst/nnstreamer/nnstreamer_log.c:97:48: error: format string is not a string literal [-Werror,-Wformat-nonliteral]
  vsnprintf (errmsg, _NNSTREAMER_ERROR_LENGTH, fmt, arg_ptr);
                                               ^~~
/Applications/Xcode_13.2.1.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/secure/_stdio.h:75:63: note: expanded from macro 'vsnprintf'
  __builtin___vsnprintf_chk (str, len, 0, __darwin_obsz(str), format, ap)
                                                              ^~~~~~
1 error generated.

Add a workaround from
https://stackoverflow.com/questions/20167124/vsprintf-and-vsnprintf-wformat-nonliteral-warning-on-clang-5-0

Note: gst-indent is applying indentation very strangely with __attribute__

Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>