events: add missing return type

The missing return type causes it to be defaulted to `int`,
which subsequently results in a compilation error if
`-Werror=return-type` or similar is set.

Fixes: 9da7bdb ("events: add placeholder function instead of #if")

events.c

@@ -120,7 +120,7 @@ static void emit_keyboard_led_hw_changed(void)
 	up_read(&leds_list_lock);
 }
 #else
-static inline emit_keyboard_led_hw_changed(void)
+static inline void emit_keyboard_led_hw_changed(void)
 { }
 #endif