Add missing <string.h> for openssl_hostname_validation module

Now it included by openssl, but nfter
openssl/openssl@master-post-reformat-1494-g6329b60 it will print warning
(apparently they dropped <string.h> from the generic headers).

sample/openssl_hostname_validation.c

@@ -41,6 +41,7 @@ SOFTWARE.
 
 #include <openssl/x509v3.h>
 #include <openssl/ssl.h>
+#include <string.h>
 
 #include "openssl_hostname_validation.h"
 #include "hostcheck.h"