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>