NFC: Add missing RFKILL dependency for Kconfig

Since the NFC subsystem gained RFKILL support, it needs to be able
to build properly with whatever option for RFKILL has been selected.

on i386:

net/built-in.o: In function `nfc_unregister_device':
(.text+0x6a36d): undefined reference to `rfkill_unregister'
net/built-in.o: In function `nfc_unregister_device':
(.text+0x6a378): undefined reference to `rfkill_destroy'
net/built-in.o: In function `nfc_register_device':
(.text+0x6a493): undefined reference to `rfkill_alloc'
net/built-in.o: In function `nfc_register_device':
(.text+0x6a4a4): undefined reference to `rfkill_register'
net/built-in.o: In function `nfc_register_device':
(.text+0x6a4b3): undefined reference to `rfkill_destroy'
net/built-in.o: In function `nfc_dev_up':
(.text+0x6a8e8): undefined reference to `rfkill_blocked'

when CONFIG_RFKILL=m but NFC is builtin.
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

net/nfc/Kconfig

@@ -4,6 +4,7 @@
 
 menuconfig NFC
 	depends on NET
+	depends on RFKILL || !RFKILL
 	tristate "NFC subsystem support"
 	default n
 	help