watchdog: move nmi_watchdog sysctl into .rodata

watchdog: move nmi_watchdog sysctl into .rodata

Move nmi_watchdog into the watchdog_sysctls array to prevent it from
unnecessary modification. This move effectively moves it inside the
.rodata section.

Initially moved out into its own non-const array in commit 9ec272c586b0
("watchdog/hardlockup: keep kernel.nmi_watchdog sysctl as 0444 if probe
fails"), which made it writable only when watchdog_hardlockup_available
was true. Moving it back to watchdog_sysctl keeps this behavior as
writing to nmi_watchdog still fails when watchdog_hardlockup_available
is false.

Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Joel Granados <joel.granados@kernel.org>