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

drivers/usb/core/Makefile

@@ -5,8 +5,9 @@
 usbcore-y := usb.o hub.o hcd.o urb.o message.o driver.o
 usbcore-y += config.o file.o buffer.o sysfs.o endpoint.o
 usbcore-y += devio.o notify.o generic.o quirks.o devices.o
-usbcore-y += port.o of.o
+usbcore-y += port.o
 
+usbcore-$(CONFIG_OF)		+= of.o
 usbcore-$(CONFIG_PCI)		+= hcd-pci.o
 usbcore-$(CONFIG_ACPI)		+= usb-acpi.o
 

drivers/usb/core/of.c

@@ -18,6 +18,7 @@
  */
 
 #include <linux/of.h>
+#include <linux/usb/of.h>
 
 /**
  * usb_of_get_child_node - Find the device node match port number