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.seSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/Makefile

@@ -35,6 +35,7 @@ obj-$(CONFIG_USB_R8A66597_HCD)	+= host/
 obj-$(CONFIG_USB_FSL_USB2)	+= host/
 obj-$(CONFIG_USB_FOTG210_HCD)	+= host/
 obj-$(CONFIG_USB_MAX3421_HCD)	+= host/
+obj-$(CONFIG_USB_XEN_HCD)	+= host/
 
 obj-$(CONFIG_USB_C67X00_HCD)	+= c67x00/