vgabios: Define PlatformRunningOn to make compile happy.

With CONFIG_COREBOOT=y, CONFIG_QEMU_HARDWARE=y, CONFIG_VGA_COREBOOT=y,
CONFIG_DEBUG_IO=y the compile would break because this variable was
missing.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
diff --git a/vgasrc/vgainit.c b/vgasrc/vgainit.c
index 6d9a224..9d6dd1e 100644
--- a/vgasrc/vgainit.c
+++ b/vgasrc/vgainit.c
@@ -32,6 +32,9 @@
 
 struct VideoParam_s video_param_table[29] VAR16;
 
+// Type of emulator platform - for dprintf with certain compile options.
+int PlatformRunningOn VAR16;
+
 
 /****************************************************************
  * PCI Data