alpha: fix compile errors in dma-mapping-common.h

The commit c186caca leads to the
following errors with CONFIG_ALPHA_{TSUNAMI, TITAN, RAWHIDE, MARVEL}:
/home/fujita/git/linux-2.6/include/asm-generic/dma-mapping-common.h: In
function `dma_map_sg_attrs':
/home/fujita/git/linux-2.6/include/asm-generic/dma-mapping-common.h:49:
error: implicit declaration of function `for_each_sg'
/home/fujita/git/linux-2.6/include/asm-generic/dma-mapping-common.h:50:
error: syntax error before "kmemcheck_mark_initialized"

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Reported-by: Michael Cree <mcree@orcon.net.nz>
Signed-off-by: Matt Turner <mattst88@gmail.com>