dmaengine: PL08x: Fix trivial build error

Something changed during the 3.1 merge window in the include files
which now causes the pl08x DMA engine driver to fail to build.  Fix
this by adding the now necessary dma-mapping.h include:

drivers/dma/amba-pl08x.c: In function ■pl08x_unmap_buffers■:
drivers/dma/amba-pl08x.c:1524: error: implicit declaration of function ■dma_unmap_single■
drivers/dma/amba-pl08x.c:1527: error: implicit declaration of function ■dma_unmap_page■

Acked-by: Vinod Koul <vinod.koul@intel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>