MAINTAINERS: USB: add uapi header files to USB SUBSYSTEM entry

For some reason the include/uapi/linux/usb/ directory is missing in the
USB SUBSYSTEM entry, so get_maintainer will not know to cc: the proper
mailing lists.  Fix this up by adding an entry for this directory.

Message-ID: <20230615094306.2072827-2-gregkh@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>