m68k: Export cachectl.h

libffi in GCC 4.2 needs cachectl.h to do its cache flushing.  But we
don't currently export it.  I believe this patch should do the trick.

Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>