add hw/serialio.c to SRC32SEG

Fixes build failure for CONFIG_COREBOOT=y.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
diff --git a/Makefile b/Makefile
index 8a3c9d1..cb15217 100644
--- a/Makefile
+++ b/Makefile
@@ -42,7 +42,7 @@
     fw/coreboot.c fw/lzmadecode.c fw/csm.c fw/biostables.c \
     fw/paravirt.c fw/shadow.c fw/pciinit.c fw/smm.c fw/mtrr.c fw/xen.c \
     fw/acpi.c fw/mptable.c fw/pirtable.c fw/smbios.c fw/romfile_loader.c
-SRC32SEG=string.c output.c pcibios.c apm.c stacks.c hw/pci.c
+SRC32SEG=string.c output.c pcibios.c apm.c stacks.c hw/pci.c hw/serialio.c
 DIRS=src src/hw src/fw vgasrc
 
 # Default compiler flags