xen-pcifront: Remove duplicate inclusion of headers.

In drivers/pci/xen-pcifront.c the xen/xenbus.h header is included twice -
once is enough.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>