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

example/x11.c

@@ -3,6 +3,8 @@
  * Sample showing how to makes SSH2 with X11 Forwarding works.
  *
  * $ ./x11 host user password [DEBUG]
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
  */
 
 #include "libssh2_setup.h"