LICENSES: Add license text files, add missing notices, fix SPDX tags

SPDX tags don't replace license files. Some notices were missing and
some tags were not according to the SPDX specification, too.

Now reuse --lint from the REUSE tool (https://reuse.software/) passes.

Reported-by: Martin Hauke <mardnh@gmx.de>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

siphash.h

@@ -1,3 +1,8 @@
+/* SPDX-License-Identifier: AGPL-3.0-or-later
+ * Copyright (c) 2021 Red Hat GmbH
+ * Author: Stefano Brivio <sbrivio@redhat.com>
+ */
+
 uint64_t siphash_8b(const uint8_t *in, const uint64_t *k);
 uint32_t siphash_12b(const uint8_t *in, const uint64_t *k);
 uint64_t siphash_20b(const uint8_t *in, const uint64_t *k);