Fix declarations of non-global variables

This patch inserts the `static` keyword to non-global variables,
which where found by the analysis tool smatch.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Closes #13970

module/os/linux/spl/spl-generic.c

@@ -90,7 +90,7 @@ EXPORT_SYMBOL(p0);
  * and use them when in_interrupt() from linux/preempt_mask.h evaluates to
  * true.
  */
-void __percpu *spl_pseudo_entropy;
+static void __percpu *spl_pseudo_entropy;
 
 /*
  * spl_rand_next()/spl_rand_jump() are copied from the following CC-0 licensed