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>

include/linux/ssb/ssb.h

@@ -7,6 +7,7 @@
 #include <linux/spinlock.h>
 #include <linux/pci.h>
 #include <linux/mod_devicetable.h>
+#include <linux/dma-mapping.h>
 
 #include <linux/ssb/ssb_regs.h>