Fix gcrypt warning when building win32 application

We must defined all members to NULL (select, waitpid, ...)

In file included from gnutls.c:63:
../../include/vlc_gcrypt.h:81: warning: missing initializer
../../include/vlc_gcrypt.h:81: warning: (near initialization for 'gcry_threads_vlc.read')
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

include/vlc_gcrypt.h

@@ -76,7 +76,8 @@ static const struct gcry_thread_cbs gcry_threads_vlc =
     gcry_vlc_mutex_init,
     gcry_vlc_mutex_destroy,
     gcry_vlc_mutex_lock,
-    gcry_vlc_mutex_unlock
+    gcry_vlc_mutex_unlock,
+    NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL
 };
 #endif