media: leds: as3645a: add V4L2_FLASH_LED_CLASS dependency

We get a link error when V4L2_FLASH_LED_CLASS=m and AS3645A is built-in:

drivers/leds/leds-as3645a.o: In function `as3645a_v4l2_setup':
leds-as3645a.c:(.text+0x258): undefined reference to `v4l2_flash_init'
leds-as3645a.c:(.text+0x284): undefined reference to `v4l2_flash_indicator_init'
leds-as3645a.c:(.text+0x2a4): undefined reference to `v4l2_flash_release'
drivers/leds/leds-as3645a.o: In function `as3645a_remove':
leds-as3645a.c:(.text+0x784): undefined reference to `v4l2_flash_release'

This adds the same Kconfig dependency that the other V4L2 flash
drivers in drivers/leds use, to avoid that broken configuration.

Fixes: a56ba8fb ("media: leds: as3645a: Add LED flash class driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>