Use apt-file
to search through the contents of packages.
To install and build the file index:
sudo apt-get install apt-file
sudo apt-file update
To search:
apt-file search predefs.h
That will give you a package name and an absolute path to where the file would be installed.
If that doesn't return anything, then search through the Ubuntu package directory using the package contents search (make sure you have your version selected in the "Distribution" field).
If that didn't turn up anything, my last piece of advice would be to Google it (I.e. "Ubuntu trusty predefs.h package) - although, I should warn you, what you find may not be as secure as the other two methods.
sudo apt-get install torcs