usb: fix host-stub.c build race

Suggested-by: NThomas Huth <thuth@redhat.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Reviewed-by: NThomas Huth <thuth@redhat.com>
Message-id: 20171004125210.7817-1-kraxel@redhat.com

hw/usb/Makefile.objs

@@ -47,6 +47,7 @@ common-obj-y += host-libusb.o host-legacy.o
 else
 common-obj-y += host-stub.o
 endif
+common-obj-$(CONFIG_ALL) += host-stub.o
 
 host-libusb.o-cflags := $(LIBUSB_CFLAGS)
 host-libusb.o-libs := $(LIBUSB_LIBS)