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>

arch/m68k/amiga/config.c

@@ -51,6 +51,8 @@ unsigned long amiga_chipset;
 EXPORT_SYMBOL(amiga_chipset);
 
 unsigned char amiga_vblank;
+EXPORT_SYMBOL(amiga_vblank);
+
 unsigned char amiga_psfreq;
 
 struct amiga_hw_present amiga_hw_present;