COMP: Fix parallel build associating patch step with "python-source" project

It is not clear why associating the PATCH_COMMAND with the python
project caused issue when doing parallel build using Ninja on Windows.
The patch step was executed before the download of python could finish.

This commit "resolves" the problem by associated the patch command
with the python source project.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22834 3bd1e089-480b-0410-8dfb-8563597acbee