usb: phy: fix phy-qcom-8x16-usb build

Fix build errors that happen when USB_QCOM_8X16_PHY=y and EXTCON=m:

drivers/built-in.o: In function `phy_8x16_init':
phy-qcom-8x16-usb.c:(.text+0x86ef4): undefined reference to `extcon_get_cable_state'
drivers/built-in.o: In function `phy_8x16_probe':
phy-qcom-8x16-usb.c:(.text+0x870bf): undefined reference to `extcon_get_edev_by_phandle'
phy-qcom-8x16-usb.c:(.text+0x87133): undefined reference to `extcon_register_interest'
phy-qcom-8x16-usb.c:(.text+0x87151): undefined reference to `extcon_unregister_interest'
drivers/built-in.o: In function `phy_8x16_remove':
phy-qcom-8x16-usb.c:(.text+0x872ec): undefined reference to `extcon_unregister_interest'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Felipe Balbi <balbi@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>