Add missed function declaration

keyboard/smart68/backlight.c

@@ -25,6 +25,8 @@ static const uint8_t backlight_table[] PROGMEM = {
     0, 16, 128, 255
 };
 
+inline void backlight_set_raw(uint8_t raw);
+
 /* Backlight pin configuration
  * PWM: PB7
  */