arch/x86: select RELOCATABLE_MODULES when POSTCAR is selected

The postcar relies on the relocatable modules support. Specifically
select that dependency.

Change-Id: If19c39c3f153cd5a526fdad6fe09b8c309ef024f
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/20635
Reviewed-by: Subrata Banik <subrata.banik@intel.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Lijian Zhao <lijian.zhao@intel.com>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/arch/x86/Kconfig

@@ -208,6 +208,7 @@ config VERSTAGE_ADDR
 config POSTCAR_STAGE
 	def_bool n
 	select NO_CAR_GLOBAL_MIGRATION
+	select RELOCATABLE_MODULES
 
 config VERSTAGE_DEBUG_SPINLOOP
 	bool