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>
diff --git a/src/southbridge/intel/i82801gx/Kconfig b/src/southbridge/intel/i82801gx/Kconfig
index 5053a1f..b549712 100644
--- a/src/southbridge/intel/i82801gx/Kconfig
+++ b/src/southbridge/intel/i82801gx/Kconfig
@@ -32,6 +32,7 @@
 	select INTEL_HAS_TOP_SWAP
 	select SOUTHBRIDGE_INTEL_COMMON_SMM
 	select SOUTHBRIDGE_INTEL_COMMON_WATCHDOG
+	select SOUTHBRIDGE_INTEL_COMMON_RTC
 
 if SOUTHBRIDGE_INTEL_I82801GX