Fix GnuTLS includes

* src/gnutls.c [HAVE_GNUTLS]: Don’t include gnutls/gnutls.h
a second time; although it doesn’t hurt, it’s not needed.
* src/sysdep.c [!HAVE_GNUTLS]: Don’t include gnutls/crypto.h,
as it may not be available.  Problem reported by Glenn Morris in:
http://lists.gnu.org/archive/html/emacs-devel/2016-08/msg00100.html

src/gnutls.c

@@ -26,7 +26,6 @@ along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
 #include "coding.h"
 
 #ifdef HAVE_GNUTLS
-#include <gnutls/gnutls.h>
 
 #ifdef WINDOWSNT
 #include <windows.h>

src/sysdep.c

@@ -95,7 +95,7 @@ along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
 #include "gnutls.h"
 /* MS-Windows loads GnuTLS at run time, if available; we don't want to
    do that during startup just to call gnutls_rnd.  */
-#ifndef WINDOWSNT
+#if defined HAVE_GNUTLS && !defined WINDOWSNT
 # include <gnutls/crypto.h>
 #else
 # define emacs_gnutls_global_init() Qnil