[PATCH] alpha: dma-mapping.h: add "struct scatterlist;"

On alpha-jensen:

  CC      drivers/base/platform.o
In file included from include/linux/dma-mapping.h:24,
                 from drivers/base/platform.c:16:
include/asm/dma-mapping.h:36: warning: "struct scatterlist" declared inside parameter list
include/asm/dma-mapping.h:36: warning: its scope is only this definition or declaration, which is probably not what you want

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>