kirkwood: add missing symbol that broke full builds

Signed-off-by: John Crispin <[email protected]>

SVN-Revision: 44641

target/linux/kirkwood/config-3.18

@@ -306,6 +306,7 @@ CONFIG_USB_EHCI_PCI=y
 CONFIG_USB_STORAGE=y
 CONFIG_USB_SUPPORT=y
 # CONFIG_USB_UHCI_HCD is not set
+# CONFIG_USB_XHCI_MVEBU is not set
 CONFIG_USE_OF=y
 CONFIG_VECTORS_BASE=0xffff0000
 # CONFIG_VFP is not set