igc: fix LEDS_CLASS dependency

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: ea57870 ("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.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/intel/Kconfig

@@ -372,6 +372,7 @@ config IGC
 config IGC_LEDS
 	def_bool LEDS_TRIGGER_NETDEV
 	depends on IGC && LEDS_CLASS
+	depends on LEDS_CLASS=y || IGC=m
 	help
 	  Optional support for controlling the NIC LED's with the netdev
 	  LED trigger.