cmake: replace PYTHON_INCLUDE_PATH with PYTHON_INCLUDE_DIR

VTK's FindPythonLibs.cmake removed this (deprecated) variable.

Change-Id: Ie945e26110dcac46c773916ac8573d0fb0cfd634
Merge topic 'remove-python-include-path' into next

820e74b1 cmake: replace PYTHON_INCLUDE_PATH with PYTHON_INCLUDE_DIR