MAINTAINERS: Add RCU's public git tree

RCU now has a public git tree, so add it via the T: line in the
MAINTAINERS file.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: joe@perches.com
LKML-Reference: <20100825155850.GA6601@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>