MAINTAINERS: add missing USB entry

Signed-off-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: NThomas Huth <thuth@redhat.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

MAINTAINERS

@@ -999,6 +999,7 @@ F: docs/usb2.txt
 F: docs/usb-storage.txt
 F: include/hw/usb.h
 F: include/hw/usb/
+F: default-configs/usb.mak
 
 USB (serial adapter)
 M: Gerd Hoffmann <kraxel@redhat.com>