update pdal-python submodules

SuperBuild/cmake/External-PDALPython.cmake

@@ -19,7 +19,7 @@ ExternalProject_Add(${_proj_name}
   GIT_REPOSITORY    https://github.com/PDAL/python
   GIT_TAG           6791a880a87e95f7318e99acfb4a10186379c5dd
   #--Update/Patch step----------
-  UPDATE_COMMAND    ""
+  UPDATE_COMMAND    git submodule update --init --recursive
   #--Configure step-------------
   SOURCE_DIR        ${SB_SOURCE_DIR}/${_proj_name}
   CMAKE_ARGS