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>