checker: require unsafe for free()

vlib/clipboard/clipboard.v

@@ -23,7 +23,9 @@ pub fn (mut cb Clipboard) clear_all() {
 
 // destroy destroys the clipboard and frees its resources.
 pub fn (mut cb Clipboard) destroy() {
-	cb.free()
+	unsafe {
+		cb.free()
+	}
 }
 
 // check_ownership returns `true` if the `Clipboard` has the content ownership.