Update msg_target_template to note auto-generation

Author: TheBlueMatt

fuzz/fuzz_targets/msg_targets/msg_target_template.txt

@@ -1,3 +1,6 @@
+// This file is auto-generated by gen_target.sh based on msg_target_template.txt
+// To modify it, modify msg_target_template.txt and run gen_target.sh instead.
+
 extern crate lightning;
 
 use lightning::ln::msgs;