USB: xen-hcd: Traverse host/ when CONFIG_USB_XEN_HCD is selected

If no other USB HCDs are selected when compiling a small pure virutal
machine, the Xen HCD driver cannot be built.

Fix it by traversing down host/ if CONFIG_USB_XEN_HCD is selected.

Fixes: 494ed399

 ("usb: Introduce Xen pvUSB frontend (xen hcd)")
Cc: stable@vger.kernel.org # v5.17+
Signed-off-by: John Ernberg <john.ernberg@actia.se>
Link: https://lore.kernel.org/r/20240517114345.1190755-1-john.ernberg@actia.se
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>