Input: add privacy screen toggle keycode

Add keycode for toggling electronic privacy screen to the keycodes
definition. Some new laptops have a privacy screen which can be toggled
with a key on the keyboard.
Signed-off-by: Mathew King <mathewk@chromium.org>
Link: https://lore.kernel.org/r/20191017163208.235518-1-mathewk@chromium.orgSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

include/uapi/linux/input-event-codes.h

@@ -649,6 +649,8 @@
  */
 #define KEY_DATA			0x277
 #define KEY_ONSCREEN_KEYBOARD		0x278
+/* Electronic privacy screen control */
+#define KEY_PRIVACY_SCREEN_TOGGLE	0x279
 
 #define BTN_TRIGGER_HAPPY		0x2c0
 #define BTN_TRIGGER_HAPPY1		0x2c0