avr32: add generic vga.h to Kbuild

Need add generic "vga.h", or can not pass building for allmodconfig,
the related error:

    CC [M]  drivers/gpu/drm/drm_irq.o
  In file included from include/linux/vgaarb.h:34,
                   from drivers/gpu/drm/drm_irq.c:42:
  include/video/vga.h:22:21: error: asm/vga.h: No such file or directory

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Acked-by: Hans-Christian Egtvedt <hegtvedt@cisco.com>