Make previous auth-source change not break on Windows without gnutls

* lisp/auth-source.el (auth-source--obfuscate)
(auth-source--deobfuscate): Check that gnutls is really available.
1 job for master in 80 minutes and 14 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #3376
test-all

01:20:14