Include stdarg.h for va_start and va_end

Fixes "implicit declaration of function" error with devtoolset-10
(gcc (GCC) 10.2.1 20210130 (Red Hat 10.2.1-11)) in centos7 Docker image
used for building an AppImage.

See also the original source code:
https://github.com/jlaurens/synctex/blob/2020/synctex_parser.c#L104