refperf: Make functions static

Because the reset_readers() and process_durations() functions are used
only within kernel/rcu/refperf.c, this commit makes them static.
Reported-by: kbuild test robot <lkp@intel.com>
Cc: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/refperf.c

@@ -283,7 +283,7 @@ ref_perf_reader(void *arg)
 	return 0;
 }
 
-void reset_readers(void)
+static void reset_readers(void)
 {
 	int i;
 	struct reader_task *rt;
@@ -296,7 +296,7 @@ void reset_readers(void)
 }
 
 // Print the results of each reader and return the sum of all their durations.
-u64 process_durations(int n)
+static u64 process_durations(int n)
 {
 	int i;
 	struct reader_task *rt;