Fix string.h, strings.h and stdio.h include in crypto backend.

String.h and stdbool.h are already included in main backend header,
no need to include them again.

Stdio.h is missing for OpenSSL and NSS backed (for sprintf).

Strings.h is missing for cipher_generic, gcrypt and OpoenSSL (strcasecmp).

Fixes: #885

lib/crypto_backend/crypto_cipher_kernel.c

@@ -19,10 +19,8 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
-#include <string.h>
 #include <stdlib.h>
 #include <stdio.h>
-#include <stdbool.h>
 #include <errno.h>
 #include <unistd.h>
 #include <sys/socket.h>