[IA64] remove duplicated #include from pci-dma.c

Removed duplicated #include <asm/machvec.h> and <linux/string.h> in
arch/ia64/kernel/pci-dma.c.

Signed-off-by: Huang Weiyi <weiyi.huang@gmail.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>