provide SPDX identifiers

- All files have prominent copyright and SPDX identifier
- If not embedded in the file, in the .reuse/dep5 file
- All used licenses are in LICENSES/ (not shipped in tarballs)
- A new REUSE CI job verify that all files are OK

Assisted-by: Viktor Szakats

Closes #1084

include/libssh2_publickey.h

@@ -41,6 +41,8 @@
  *
  * For more information on the publickey subsystem,
  * refer to IETF draft: secsh-publickey
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
  */
 
 #ifndef LIBSSH2_PUBLICKEY_H