Add copyright notices

Only on the files where I have contributed in a significant way,
or the files I wrote myself.

examples/imatrix/imatrix.cpp

@@ -1,3 +1,10 @@
+//
+// Copyright (C) 2024 Iwan Kawrakow
+// Copyright (C) 2023-2024 The ggml authors
+// MIT license
+// SPDX-License-Identifier: MIT
+//
+
 #include "common.h"
 #include "llama.h"