kernel: fix implicit use of kmod.h in power/suspend.c

kernel: fix implicit use of kmod.h in power/suspend.c

File power/suspend.c was implicitly relying on <linux/kmod.h>
coming in via module.h, as without it we get:

kernel/power/suspend.c: In function ‘suspend_prepare’:
kernel/power/suspend.c:100: error: implicit declaration of function ‘usermodehelper_disable’
kernel/power/suspend.c:109: error: implicit declaration of function ‘usermodehelper_enable’

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>