arch/x86: Fix building with CONFIG_VBOOT_SEPARATE_VERSTAGE=n

TESTED on qemu/i440fx with VBOOT_SEPARATE_VERSTAGE commented out.

Change-Id: I1227feab9ffbbc06e614f297be44fb67f13bda42
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/52784
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/src/arch/x86/Makefile.inc b/src/arch/x86/Makefile.inc
index 7dea2ce..a3c61c4 100644
--- a/src/arch/x86/Makefile.inc
+++ b/src/arch/x86/Makefile.inc
@@ -101,7 +101,7 @@
 
 ifeq ($(CONFIG_ARCH_VERSTAGE_X86_32)$(CONFIG_ARCH_VERSTAGE_X86_64),y)
 
-verstage-y += assembly_entry.S
+verstage-$(CONFIG_VBOOT_SEPARATE_VERSTAGE) += assembly_entry.S
 verstage-y += boot.c
 verstage-y += post.c
 verstage-$(CONFIG_VBOOT_SEPARATE_VERSTAGE) += gdt_init.S