usb: fix libusb config variable name.

Cc: Jan Kiszka <jan.kiszka@siemens.com>
Fixes: 4e5ee5b
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Tested-by: Jan Kiszka <jan.kiszka@siemens.com>
Message-id: 20170926063820.30773-1-kraxel@redhat.com

hw/usb/Makefile.objs

@@ -42,7 +42,7 @@ redirect.o-cflags = $(USB_REDIR_CFLAGS)
 redirect.o-libs = $(USB_REDIR_LIBS)
 
 # usb pass-through
-ifeq ($(CONFIG_LIBUSB)$(CONFIG_USB),yy)
+ifeq ($(CONFIG_USB_LIBUSB)$(CONFIG_USB),yy)
 common-obj-y += host-libusb.o host-legacy.o
 else
 common-obj-y += host-stub.o