openrisc: include: asm: Kbuild: add default "vga.h"

Add default "vga.h", or compiling fails with 'allmodconfig'.

The related error:

    CC [M]  drivers/gpu/drm/drm_irq.o
  In file included from include/linux/vgaarb.h:34:0,
                   from drivers/gpu/drm/drm_irq.c:42:
  include/video/vga.h:22:21: fatal error: asm/vga.h: No such file or directory
  compilation terminated.
  make[3]: *** [drivers/gpu/drm/drm_irq.o] Error 1
  make[2]: *** [drivers/gpu/drm] Error 2
  make[1]: *** [drivers/gpu] Error 2
  make: *** [drivers] Error 2

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Jonas Bonn <jonas@southpole.se>