GPGPU: Shorten ppcg include paths to avoid conflict with cuda.h

Instead of directly linking to ppcg's main source directory, we link to the
parent director. This allows us to access ppcg's include files with
'ppcg/cuda.h' and avoids a conflict with NVIDIA's cuda.h header.

Also drop an include directory that is currently not used.

llvm-svn: 275536