arch: hexagon: include: asm: add "vga.h" in Kbuild

Need include generic "vga.h", or can not pass compiling 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

Also move "preempt.h" upper to match sort order.

Signed-off-by: Chen Gang <[e-mail]>
Signed-off-by: Richard Kuo <[e-mail]>

arch/hexagon/include/asm/Kbuild

···
 generic-y += percpu.h
 generic-y += poll.h
 generic-y += posix_types.h
+generic-y += preempt.h
 generic-y += resource.h
 generic-y += rwsem.h
 generic-y += scatterlist.h
···
 generic-y += types.h
 generic-y += ucontext.h
 generic-y += unaligned.h
+generic-y += vga.h
 generic-y += xor.h
-generic-y += preempt.h