include/linux/vgaarb.h: add missing part of include guard

vgaarb.h was missing the #define of the #ifndef at the top for the guard
to prevent multiple #include's from causing re-define errors

Signed-off-by: Doug Goldstein <[e-mail]>
Cc: Dave Airlie <[e-mail]>
Cc: Jesse Barnes <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Dave Airlie <[e-mail]>

include/linux/vgaarb.h

···
  */
 
 #ifndef LINUX_VGA_H
+#define LINUX_VGA_H
 
 #include <asm/vga.h>