Fixes for building with Ryzen AI Software 1.3.0

[fifield]

Two changes here:

1. The Ryzen AI Software 1.3.0 installer installs things into a python venv. The binaries are put in the venv binary path (e.g. venv/bin) whereas the libraries and headers are installed into the python site-packages directory (e.g.venv/lib/python3.10/site-packages/include). This means that just using the path of xchesscc is not adequate for finding AIETOOLS_DIR path. So, this PR updates FindAIETools.cmake to search for adf.h to locate the include path instead of inferring it from the binary path. Then, AIETOOLS_DIR path is the parent of the include dir.

2. Make DSP lib optional because it is not present in the Ryzen AI software installation.

[fifield]

Without this I get errors like:

  [139/591] Generating mm.o
  FAILED: aie_kernels/mm.o /workspace/build/aie_kernels/mm.o 
  cd /workspace/build/aie_kernels && /workspace/mlir-aie/tools/chess-clang/xchesscc_wrapper AIE2 -aietools /workspace/venv -I /workspace/venv/include -c /workspace/mlir-aie/aie_kernels/mm.cc -o /workspace/build/aie_kernels/mm.o
  Error: Couldn't open "/workspace/venv/data/aie_ml/lib/me.prx": no such file or directory
  xchesscc Failed 
  The Program xchesscc has encountered an unexpected error exiting ... xchesscc Failed