When IGC is built-in but LEDS_CLASS is a loadable module, there is
a link failure:

x86_64-linux-ld: drivers/net/ethernet/intel/igc/igc_leds.o: in function `igc_led_setup':
igc_leds.c:(.text+0x75c): undefined reference to `devm_led_classdev_register_ext'

Add another dependency that prevents this combination.

Fixes: ea578703 ("igc: Add support for LEDs on i225/i226")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Kurt Kanzenbach <kurt@linutronix.de>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Link: https://lore.kernel.org/r/20240301184806.2634508-4-anthony.l.nguyen@intel.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>