sb/intel/common: Make linking rtc.c conditional

Change-Id: I7321da453c0d9bb4a142c3c93103d8dc0ff416b7
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/33201
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/southbridge/intel/ibexpeak/Kconfig

@@ -35,6 +35,7 @@ config SOUTH_BRIDGE_OPTIONS # dummy
 	select SOUTHBRIDGE_INTEL_COMMON_SMM
 	select SOUTHBRIDGE_INTEL_COMMON_PMCLIB
 	select SOUTHBRIDGE_INTEL_COMMON_PMBASE
+	select SOUTHBRIDGE_INTEL_COMMON_RTC
 	select HAVE_USBDEBUG_OPTIONS
 	select COMMON_FADT
 	select ACPI_SATA_GENERATOR