usb: select USB_COMMON for usb role switch config

The new axp288 extcon driver has no dependency on USB itself but
calls the usb role switch helper functions. This causes a link error
when USB_COMMON is disabled, as that subdirectory never gets entered:

drivers/extcon/extcon-axp288.o: In function `axp288_usb_role_work':
extcon-axp288.c:(.text+0x47b): undefined reference to `usb_role_switch_set_role'
extcon-axp288.c:(.text+0x498): undefined reference to `usb_role_switch_get_role'
drivers/extcon/extcon-axp288.o: In function `axp288_extcon_probe':
extcon-axp288.c:(.text+0x675): undefined reference to `usb_role_switch_get'
extcon-axp288.c:(.text+0x6d1): undefined reference to `usb_role_switch_put'
drivers/extcon/extcon-axp288.o: In function `axp288_put_role_sw':
extcon-axp288.c:(.text+0x1c): undefined reference to `usb_role_switch_put'

There are multiple ways of fixing this, I chose to 'select USB_COMMON',
since that is how we solved the same problem for other helpers like
USB_LED_TRIG or PHY drivers.

Fixes: d54f063c ("extcon: axp288: Set USB role where necessary")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/Kconfig

@@ -207,5 +207,6 @@ config USB_ULPI_BUS
 
 config USB_ROLE_SWITCH
 	tristate
+	select USB_COMMON
 
 endif # USB_SUPPORT