efi/reboot: Make function pointer orig_pm_power_off static

The function pointer orig_pm_power_off is local to the source and does
not need to be in global scope, so make it static.

Cleans up sparse warning:
symbol 'orig_pm_power_off' was not declared. Should it be static?
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Link: http://lkml.kernel.org/r/20170825155019.6740-4-ard.biesheuvel@linaro.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

drivers/firmware/efi/reboot.c

@@ -5,7 +5,7 @@
 #include <linux/efi.h>
 #include <linux/reboot.h>
 
-void (*orig_pm_power_off)(void);
+static void (*orig_pm_power_off)(void);
 
 int efi_reboot_quirk_mode = -1;