usb: only build usb-host with CONFIG_USB=y

Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Reviewed-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NThomas Huth <thuth@redhat.com>
Tested-by: NThomas Huth <thuth@redhat.com>
Message-id: 20170908111217.21985-3-kraxel@redhat.com

hw/usb/Makefile.objs

@@ -38,7 +38,7 @@ endif
 common-obj-$(CONFIG_USB_REDIR) += redirect.o quirks.o
 
 # usb pass-through
-ifeq ($(CONFIG_LIBUSB),y)
+ifeq ($(CONFIG_LIBUSB)$(CONFIG_USB),yy)
 common-obj-y += host-libusb.o host-legacy.o
 else
 common-obj-y += host-stub.o