kernel: add missing config symbol after rfkill change

Signed-off-by: Felix Fietkau <[email protected]>

target/linux/generic/config-4.4

@@ -901,6 +901,7 @@ CONFIG_DEFAULT_SECURITY=""
 CONFIG_DEFAULT_SECURITY_DAC=y
 CONFIG_DEFAULT_TCP_CONG="cubic"
 CONFIG_DEFCONFIG_LIST="/lib/modules/$UNAME_RELEASE/.config"
+# CONFIG_DELL_RBTN is not set
 # CONFIG_DELL_SMO8800 is not set
 # CONFIG_DEPRECATED_PARAM_STRUCT is not set
 # CONFIG_DETECT_HUNG_TASK is not set