watchdog/hpwdt: Include nmi.h only if CONFIG_HPWDT_NMI_DECODING

watchdog/hpwdt: Include nmi.h only if CONFIG_HPWDT_NMI_DECODING

Fixes: 05001c67d46e ("x86, nmi, drivers: Fix nmi splitup build bug")
Arm64 does not support NMI and has no <asm/nmi.h>.

Include <asm/nmi.h> only if CONFIG_HPWDT_NMI_DECODING is defined to
avoid build failure on non-existent header file on Arm64.

Signed-off-by: Jerry Hoemann <jerry.hoemann@hpe.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20220820202821.1263837-2-jerry.hoemann@hpe.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>