Fix adaptor's cmake search path for 5.6.0 release only
The 5.6.0 version lacks any CMAKE_PREFIX_PATH line, so the patch has to add it, not modify it like the last attempt tried to do. The patch is predicated on paraview_SOURCE_SELECTION == 5.6.0 so it has not effect on master or release, both of which already have the correct "${CMAKE_... line.
Backport: release