ssb: Include dma-mapping.h

ssb.h implements DMA mapping functions, so it should
include dma-mapping.h. This fixes compile failures on certain architectures.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>