hw/usb/redirect: Do not link 'usb-redir' device when USB not enabled

The 'usb-redir' device requires the USB core code to work. Do not
link it when there is no USB support. This fixes:

  $ qemu-system-tricore -M tricore_testboard -device usb-redir
  qemu-system-tricore: -device usb-redir: No 'usb-bus' bus found for device 'usb-redir'

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20191231183216.6781-2-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/usb/Makefile.objs

@@ -39,9 +39,11 @@ common-obj-$(CONFIG_USB_STORAGE_MTP)  += dev-mtp.o
 endif
 
 # usb redirection
+ifeq ($(CONFIG_USB),y)
 common-obj-$(CONFIG_USB_REDIR) += redirect.o quirks.o
 redirect.o-cflags = $(USB_REDIR_CFLAGS)
 redirect.o-libs = $(USB_REDIR_LIBS)
+endif
 
 # usb pass-through
 ifeq ($(CONFIG_USB_LIBUSB)$(CONFIG_USB),yy)