add some missing headers
I noticed there were a few missing headers while using TRooFit as a submodule of the "statistical-analysis" package: https://gitlab.cern.ch/emus/statistical-analysis
There were only two missing header files, which are added in this MR.
- "TROOT.h" inside TRooFit/apps/TRooHypoTester.h
- "TColor.h" inside src/TRooWorkspace.cxx