A five second search would tell you it's dos.h. You need to learn to find these things out quickly rather than waiting around on a forum.
Word of Advice: Don't add stuff like "Processing..." to your program for no reason. It will either come off as a silly attempt to look cute or worse, it will look like your code is buggy. Spend your time coding things that are actually useful to your end-user for now. You can make cool looking programs after you learn some GUI proramming.
It's a fairly new function, not every compiler may have it out of the box. In particular, GCC needs to be passed the commandline options -std=c++11 -D_GLIBCXX_USE_NANOSLEEP -pthread. If you're stuck with something that doesn't have it, get boost (www.boost.org if it's not already installed on your system). If you can't get boost, look at your OS API but get boost anyway.