vgabios: Place a signature at offset 0x1e of the rom.

It appears some emulators look for a signature at offset 0x1e of the
option rom.  So, port the signature from the LGPL bios.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
diff --git a/vgasrc/vgaentry.S b/vgasrc/vgaentry.S
index 2dc07c1..112857b 100644
--- a/vgasrc/vgaentry.S
+++ b/vgasrc/vgaentry.S
@@ -40,6 +40,10 @@
 #endif
 _rom_header_pnpdata:
         .word 0
+_rom_header_other2:
+        .word 0
+_rom_header_signature:
+        .asciz "IBM"
 
 
 /****************************************************************