leds: simatic-ipc-leds-gpio: Restore LEDS_CLASS dependency

A recent rework accidentally lost the dependency on LEDS_CLASS, which
leads to a link error when LED support is disbled:

x86_64-linux-ld: drivers/leds/simple/simatic-ipc-leds.o: in function `simatic_ipc_leds_probe':
simatic-ipc-leds.c:(.text+0x10c): undefined reference to `devm_led_classdev_register_ext'

Add back the dependency that was there originally.

Fixes: a6c80bec3c935 ("leds: simatic-ipc-leds-gpio: Add GPIO version of Siemens driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20230623152233.2246285-1-arnd@kernel.org
Signed-off-by: Lee Jones <lee@kernel.org>