fix compiler error in paddle:latest-dev image

[luotao1]

fix #9906
The reason is that original file(GLOB_RECURSE FLUID_PY_FILES ./paddle/fluid/ *.py) will add paddle/v2 files.

After fix #9906, there is compile error:

Error copying directory from "/Paddle/python/paddle" to 
"/Paddle/docker_build/python/paddle".
python/CMakeFiles/paddle_python.dir/build.make:352: recipe for target 
'python/build/.timestamp' failed
make[2]: *** [python/build/.timestamp] Error 1
CMakeFiles/Makefile2:30120: recipe for target 'python/CMakeFiles/paddle_python.dir/all' 
failed
make[1]: *** [python/CMakeFiles/paddle_python.dir/all] Error 2
Makefile:160: recipe for target 'all' failed
make: *** [all] Error 2

Thus, change COMMAND ${CMAKE_COMMAND} -E copy_directory to COMMAND cp -r

[wangkuiyi]

Thanks for this fix!