Insert, not append explicit xcbgen dir python path

Insert, not append explicit xcbgen dir python path

If a the path to the xcb python generate libs is
explicitly specified to c_client.py, insert it in
the python path list just after the local dir entry,
rather than appending it to the existing paths.
This keeps a global/distro install of xcb from
overriding a local build of the xcb proto files.

Signed-off-by: James Jones <jajones@nvidia.com>
Signed-off-by: Jamey Sharp <jamey@minilop.net>