staging: r8723au: Fix build problem when RFKILL is not selected

The kbuild test robot reports the following build errors for x86_64-randconfig-c1-0407:

All error/warnings:

   net/built-in.o: In function `wiphy_new':
>> (.text+0x7684c): undefined reference to `rfkill_alloc'
   net/built-in.o: In function `wiphy_rfkill_start_polling':
>> (.text+0x76da4): undefined reference to `rfkill_resume_polling'
   net/built-in.o: In function `wiphy_rfkill_stop_polling':
>> (.text+0x76de9): undefined reference to `rfkill_pause_polling'
   net/built-in.o: In function `wiphy_unregister':
>> (.text+0x76ec9): undefined reference to `rfkill_unregister'
   net/built-in.o: In function `wiphy_rfkill_set_hw_state':
>> (.text+0x771bc): undefined reference to `rfkill_set_hw_state'
   net/built-in.o: In function `wiphy_register':
>> (.text+0x77968): undefined reference to `rfkill_register'
   net/built-in.o: In function `wiphy_register':
>> (.text+0x77981): undefined reference to `rfkill_destroy'
   net/built-in.o: In function `cfg80211_rfkill_sync_work':
>> core.c:(.text+0x788ad): undefined reference to `rfkill_blocked'
   net/built-in.o: In function `cfg80211_dev_free':
>> (.text+0x78a7b): undefined reference to `rfkill_destroy'
   net/built-in.o: In function `cfg80211_netdev_notifier_call':
>> core.c:(.text+0x79203): undefined reference to `rfkill_blocked'
   net/built-in.o: In function `nl80211_start_p2p_device':

These undefined sysbols are all satisfied if a "select RFKILL" is added to
Kconfig. This "fix" leads to another problem as follows:

>> nl80211.c:(.text+0x9032e): undefined reference to `rfkill_blocked'
net/rfkill/Kconfig:4:error: recursive dependency detected!
net/rfkill/Kconfig:4:   symbol RFKILL is selected by R8723AU
drivers/staging/rtl8723au/Kconfig:1:    symbol R8723AU depends on USB
drivers/usb/Kconfig:41: symbol USB is selected by MOUSE_APPLETOUCH
drivers/input/mouse/Kconfig:162:        symbol MOUSE_APPLETOUCH depends on INPUT
drivers/input/Kconfig:8:        symbol INPUT is selected by ACPI_CMPC
drivers/platform/x86/Kconfig:635:       symbol ACPI_CMPC depends on RFKILL

To avoid substituting one build error for another, I added a "depends on RFKILL".
My suspicion is that this particular error is caused by a kbuild bug, or that the
selection of INPUT by ACPI_CMPC is wrong. In any case, that will be solved separately.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Jes Sorensen <Jes.Sorensen@redhat.com>
Cc: kbuild-all@01.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/drivers/staging/rtl8723au/Kconfig b/drivers/staging/rtl8723au/Kconfig
index 78e8805..07fb5e4 100644
--- a/drivers/staging/rtl8723au/Kconfig
+++ b/drivers/staging/rtl8723au/Kconfig
@@ -1,6 +1,6 @@
 config R8723AU
 	tristate "Realtek RTL8723AU Wireless LAN NIC driver"
-	depends on WLAN && USB
+	depends on USB && WLAN && RFKILL
 	select WIRELESS_EXT
 	select WEXT_PRIV
 	select CFG80211