kernel.h: remove duplicate include of asm/div64.h

This seems to have been added inadvertently in commit
  72deb455 ("block: remove CONFIG_LBDAF")

Fixes: 72deb455

 ("block: remove CONFIG_LBDAF")
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: http://lkml.kernel.org/r/20200727034852.2813453-1-nivedita@alum.mit.edu
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>