sandbox: Don't use PCI in SPL

PCI is not supported in SPL for sandbox, so avoid using it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/lib/Makefile b/arch/sandbox/lib/Makefile
index 96761e2..7820c55 100644
--- a/arch/sandbox/lib/Makefile
+++ b/arch/sandbox/lib/Makefile
@@ -8,5 +8,7 @@
 #
 
 obj-y	+= interrupts.o
+ifndef CONFIG_SPL_BUILD
 obj-$(CONFIG_PCI)	+= pci_io.o
+endif
 obj-$(CONFIG_CMD_BOOTM) += bootm.o