USB: xhci: Make xhci-mem.c include linux/dmapool.h

xhci-mem.c includes calls to dma_pool_alloc() and other functions defined
in linux/dmapool.h.  Make sure to include that header file.

Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>