rcu: Remove "extern" from function declaration in include/linux/rcupdate.h

Because functions have the extern storage class specifier by default,
this keyword can be removed. It is redundant to use it explicitly.

Signed-off-by: Iulia Manda <iulia.manda21@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>