scftorture: Make symbol 'scf_torture_rand' static

The sparse tool complains as follows

kernel/scftorture.c:124:1: warning:
 symbol '__pcpu_scope_scf_torture_rand' was not declared. Should it be static?

And this per-CPU variable is not used outside of scftorture.c,
so this commit marks it static.
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/scftorture.c

@@ -134,7 +134,7 @@ static atomic_t n_alloc_errs;
 static bool scfdone;
 static char *bangstr = "";
 
-DEFINE_TORTURE_RANDOM_PERCPU(scf_torture_rand);
+static DEFINE_TORTURE_RANDOM_PERCPU(scf_torture_rand);
 
 // Print torture statistics.  Caller must ensure serialization.
 static void scf_torture_stats_print(void)