usb: core: of.c: fix defined but not declare warning

The helper usb_of_get_child_node is defined at of.c, but missing its
declare as a global function. Fix it by adding related header file
as well as compile it on conditional of CONFIG_OF.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: linux-usb@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: Ben Dooks <ben.dooks@codethink.co.uk>
Cc: linux-kernel@lists.codethink.co.uk

Signed-off-by: Peter Chen <peter.chen@nxp.com>
Reported-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>