rbtree: include rcu.h

Since commit c1adf200 ("Introduce rb_replace_node_rcu()")
rbtree_augmented.h uses RCU related data structures but does not include
the header file.  It works as long as it gets somehow included before
that and fails otherwise.

Link: http://lkml.kernel.org/r/20180504103159.19938-1-bigeasy@linutronix.de


Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>