datapath: Stop using __DATE__ and __TIME__ in startup string.

datapath: Stop using __DATE__ and __TIME__ in startup string.

An increasing number of distributions ship with GCC 4.9 (including
Fedora and Ubuntu) that has -Werror=date-time. This causes kernel
compilation to fail because the builds are not exactly reproducible.

This simply removes the use of those constants, which was already
done for the upstream Linux version of the module. It retains the
version string, however, which should provide the same information
in most cases.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>