export amiga_vblank

This patch fixes the following build error:

<--  snip  -->

..
  Building modules, stage 2.
  MODPOST 1203 modules
ERROR: "amiga_vblank" [drivers/video/amifb.ko] undefined!
..
make[2]: *** [__modpost] Error 1

<--  snip  -->

Reported-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>