mbedtls_stub: Add missing include

mbedtls_stub uses memset but doesn't add the <string.h> include. This
causes build failures with clang.

Add the include so we can build unit tests using clang.

Author: rwalton-arm

connectivity/nanostack/coap-service/test/coap-service/unittest/stub/mbedtls_stub.c

@@ -15,6 +15,7 @@
  * limitations under the License.
  */
 
+#include <string.h> // memset
 #include "mbedtls_stub.h"
 
 mbedtls_stub_def mbedtls_stub;