[media] v4l2-flash-led-class: Add missing VIDEO_V4L2 Kconfig dependency

Fixes the following randconfig problem:

drivers/built-in.o: In function `v4l2_flash_release':
(.text+0x12204f): undefined reference to `v4l2_async_unregister_subdev'
drivers/built-in.o: In function `v4l2_flash_release':
(.text+0x122057): undefined reference to `v4l2_ctrl_handler_free'
drivers/built-in.o: In function `v4l2_flash_close':
v4l2-flash-led-class.c:(.text+0x12208f): undefined reference to `v4l2_fh_is_singular'
v4l2-flash-led-class.c:(.text+0x1220c8): undefined reference to `__v4l2_ctrl_s_ctrl'
drivers/built-in.o: In function `v4l2_flash_open':
v4l2-flash-led-class.c:(.text+0x12227f): undefined reference to `v4l2_fh_is_singular'
drivers/built-in.o: In function `v4l2_flash_init_controls':
v4l2-flash-led-class.c:(.text+0x12274e): undefined reference to `v4l2_ctrl_handler_init_class'
v4l2-flash-led-class.c:(.text+0x122797): undefined reference to `v4l2_ctrl_new_std_menu'
v4l2-flash-led-class.c:(.text+0x1227e0): undefined reference to `v4l2_ctrl_new_std'
v4l2-flash-led-class.c:(.text+0x122826): undefined reference to `v4l2_ctrl_handler_setup'
v4l2-flash-led-class.c:(.text+0x122839): undefined reference to `v4l2_ctrl_handler_free'
drivers/built-in.o: In function `v4l2_flash_init':
(.text+0x1228e2): undefined reference to `v4l2_subdev_init'
drivers/built-in.o: In function `v4l2_flash_init':
(.text+0x12293b): undefined reference to `v4l2_async_register_subdev'
drivers/built-in.o: In function `v4l2_flash_init':
(.text+0x122949): undefined reference to `v4l2_ctrl_handler_free'
drivers/built-in.o:(.rodata+0x20ef8): undefined reference to `v4l2_subdev_queryctrl'
drivers/built-in.o:(.rodata+0x20f10): undefined reference to `v4l2_subdev_querymenu'

Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Sakari Ailus <sakari.ailus@iki.fi>
Cc: Hans Verkuil <hans.verkuil@cisco.com>