python: Fix "make distcheck" error on version.py.

python: Fix "make distcheck" error on version.py.

The generated version.py has to go in the srcdir and has to be regenerated
based on config.status, which breaks "make distcheck" because it
write-protects the srcdir.  However, the contents of version.py only change
when the version number changes, so we can just "touch" it when it doesn't
really need to change.

The same pattern is used elsewhere in the tree for other files in the same
situation, e.g. the various RPM spec files.

Reported-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Ben Pfaff <blp@nicira.com>