validate: Add license headers

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/1586>

gstreamer-validate/src/action_type.rs

@@ -1,3 +1,5 @@
+// Take a look at the license at the top of the repository in the LICENSE file.
+
 use std::{ffi::c_int, ptr};
 
 use crate::{ffi, Action};

gstreamer-validate/src/enums.rs

@@ -1,3 +1,5 @@
+// Take a look at the license at the top of the repository in the LICENSE file.
+
 use crate::ffi;
 use glib::translate::*;
 

gstreamer-validate/src/reporter.rs

@@ -1,3 +1,5 @@
+// Take a look at the license at the top of the repository in the LICENSE file.
+
 use glib::translate::*;
 
 use crate::action::Action;