rcu: Remove "extern" from function declarations in kernel/rcu/rcu.h

Function prototypes don't need to have the "extern" keyword since this
is the default behavior. Its explicit use is redundant.  This commit
therefore removes them.
Signed-off-by: Teodora Baluta <teobaluta@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/rcu.h

@@ -96,7 +96,7 @@ static inline void debug_rcu_head_unqueue(struct rcu_head *head)
 }
 #endif	/* #else !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
 
-extern void kfree(const void *);
+void kfree(const void *);
 
 static inline bool __rcu_reclaim(const char *rn, struct rcu_head *head)
 {