rcu: Flag need for rcu_node_tree.h and rcu_segcblist.h visibility

The rcu_node_tree.h and rcu_segcblist.h header files in the include/linux
directory might appear at first sight to be internal to the RCU
implementation.  However, the definitions in these files are needed to
determine the size of TREE SRCU's srcu_struct structure, so they must
be externally visible, which is why they live in include/linux.

This commit adds comments to this effect to those files.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>