Add stubs for VIA vga bios callbacks to system bios.

The VIA vga bios likes wants to call int 15/5f of main bios.  So, add
    stubs to make it happy.
diff --git a/Makefile b/Makefile
index 16d3882..693524b 100644
--- a/Makefile
+++ b/Makefile
@@ -10,7 +10,7 @@
 # Source files
 SRCBOTH=output.c util.c floppy.c ata.c system.c mouse.c kbd.c pci.c \
         serial.c clock.c pic.c cdrom.c ps2port.c
-SRC16=$(SRCBOTH) disk.c apm.c pcibios.c
+SRC16=$(SRCBOTH) disk.c apm.c pcibios.c vgahooks.c
 SRC32=$(SRCBOTH) post.c shadow.c post_menu.c memmap.c coreboot.c boot.c \
       acpi.c pirtable.c smm.c smpdetect.c mptable.c smbios.c pciinit.c
 TABLESRC=font.c cbt.c floppy_dbt.c