snps_udc_plat.c:undefined reference to `extcon_unregister_notifier'

[kbuild test robot]

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git 
master
head:   f007cad159e99fa2acd3b2e9364fbb32ad28b971
commit: 1b9f35adb0ffa143c7972a8459d6979c77d6c3c0 usb: gadget: udc: Add Synopsys 
UDC Platform driver
date:   3 months ago
config: x86_64-randconfig-b0-09112030 (attached as .config)
compiler: gcc-6 (Debian 6.2.0-3) 6.2.0 20160901
reproduce:
        git checkout 1b9f35adb0ffa143c7972a8459d6979c77d6c3c0
        # save the attached .config to linux build tree
        make ARCH=x86_64 

All errors (new ones prefixed by >>):

   drivers/built-in.o: In function `udc_plat_remove':
>> snps_udc_plat.c:(.text+0x2c4060): undefined reference to 
>> `extcon_unregister_notifier'
   drivers/built-in.o: In function `udc_plat_probe':
>> snps_udc_plat.c:(.text+0x2c438c): undefined reference to 
>> `extcon_get_edev_by_phandle'
>> snps_udc_plat.c:(.text+0x2c43f2): undefined reference to 
>> `extcon_register_notifier'
>> snps_udc_plat.c:(.text+0x2c4416): undefined reference to `extcon_get_state'
   snps_udc_plat.c:(.text+0x2c44f7): undefined reference to 
`extcon_unregister_notifier'

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