leds: tps6131x: Add V4L2_FLASH_LED_CLASS dependency

leds: tps6131x: Add V4L2_FLASH_LED_CLASS dependency

This driver can optionally use the v4l2_flash infrastructure, but fails to
link built=in if that is in a loadable module:

ld.lld-21: error: undefined symbol: v4l2_flash_release
>>> referenced by leds-tps6131x.c:792 (drivers/leds/flash/leds-tps6131x.c:792)

Add the usual Kconfig dependency for it, still allowing it to be built when
CONFIG_V4L2_FLASH_LED_CLASS is disabled.

Fixes: b338a2ae9b31 ("leds: tps6131x: Add support for Texas Instruments TPS6131X flash LED driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20250620114440.4080938-1-arnd@kernel.org
Signed-off-by: Lee Jones <lee@kernel.org>