As the RCU symbols are going to be changed to GPL in the near future,
lets warn users that this is going to happen.

Cc: Paul McKenney <paulmck@us.ibm.com>
Acked-by: NDipankar Sarma <dipankar@in.ibm.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>