usb: dwc3: add EXTCON dependency for qcom

Like the omap back-end, we get a link error with CONFIG_EXTCON=m
when building the qcom back-end into the kernel:

drivers/usb/dwc3/dwc3-qcom.o: In function `dwc3_qcom_probe':
dwc3-qcom.c:(.text+0x13dc): undefined reference to `extcon_get_edev_by_phandle'
dwc3-qcom.c:(.text+0x1b18): undefined reference to `devm_extcon_register_notifier'
dwc3-qcom.c:(.text+0x1b9c): undefined reference to `extcon_get_state'

Do the same thing as OMAP and add an explicit dependency on
EXTCON.

Fixes: a4333c3a ("usb: dwc3: Add Qualcomm DWC3 glue driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>