Declare functions to silence byte-compiler

Declare functions to silence byte-compiler

* lisp/auth-source.el (gnutls-symmetric-decrypt, gnutls-ciphers):
* lisp/net/dbus.el (libxml-parse-xml-region):
* lisp/simple.el (thread-name):
* lisp/thread.el (thread-name, thread-signal, thread--blocker)
(current-thread, thread-live-p, all-threads):
* test/lisp/emacs-lisp/multisession-tests.el (sqlite-close):
* test/lisp/net/gnutls-tests.el (gnutls-symmetric-decrypt)
(gnutls-symmetric-encrypt, gnutls-hash-mac, gnutls-hash-digest)
(gnutls-ciphers, gnutls-digests, gnutls-macs):
* test/lisp/net/network-stream-tests.el (gnutls-peer-status):
* test/lisp/net/shr-tests.el (libxml-parse-html-region):
* test/src/decompress-tests.el (zlib-decompress-region):
* test/src/process-tests.el (thread-last-error, thread-join)
(make-thread):
* test/src/xml-tests.el (libxml-parse-xml-region): Declare
functions to silence byte-compiler in --without-all builds.