arm: fix implicit usage of string.h in kernel/leds.c

arm: fix implicit usage of string.h in kernel/leds.c

The use of these string ops was implicitly hidden by the use
of module.h, but that is going away shortly.  When it does, this
will break as follows:

arch/arm/kernel/leds.c: In function 'leds_store':
arch/arm/kernel/leds.c:40: error: implicit declaration of function 'strcspn'
arch/arm/kernel/leds.c:40: warning: incompatible implicit declaration of built-in function 'strcspn'
arch/arm/kernel/leds.c:45: error: implicit declaration of function 'strncmp'
arch/arm/kernel/leds.c:55: error: implicit declaration of function 'strlen'
arch/arm/kernel/leds.c:55: warning: incompatible implicit declaration of built-in function 'strlen'
make[2]: *** [arch/arm/kernel/leds.o] Error 1

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