RDMA/cxgb3: Fix build on sparc64

cxgb3 uses dma_alloc_coherent() et al. thus needs linux/dma-mapping.h
include in order to build reliably.

Noticed on sparc64.

Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>