printk: make suppress_panic_printk static

This symbol is not used outside of printk.c, so marks it static.

Fix the following sparse warning:

kernel/printk/printk.c:100:19: warning: symbol 'suppress_panic_printk'
was not declared. Should it be static?
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20220216031957.9761-1-jiapeng.chong@linux.alibaba.com

kernel/printk/printk.c

@@ -97,7 +97,7 @@ int __read_mostly suppress_printk;
  * During panic, heavy printk by other CPUs can delay the
  * panic and risk deadlock on console resources.
  */
-int __read_mostly suppress_panic_printk;
+static int __read_mostly suppress_panic_printk;
 
 #ifdef CONFIG_LOCKDEP
 static struct lockdep_map console_lock_dep_map = {