llpc/CMakeLists.txt: remove redundant PROJECT_SOURCE_DIR
llpc/CMakeLists.txt: remove redundant PROJECT_SOURCE_DIR

Since project() is called in this CMakeLists.txt file, relative paths
should work just fine.

GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc