Python build: copy files instead of creating link (#1989)

After https://github.com/iovisor/bcc/pull/1826

 version.py is generated
by cmake in order to have the __version__ define in the module. The
previous approach of installing the python module created a link between the
src and the "build" folder, hence the version.py was actually created in the
scr folder, this caused some git noise.

This comit solves that problem by copying files instead of creating a link,
then the version.py file is only created in the "build" dir.

Signed-off-by: Mauricio Vasquez B <mauriciovasquezbernal@gmail.com>