lib509: add missing include for strdup

Closes https://github.com/curl/curl/pull/3739

tests/libtest/lib509.c

@@ -21,6 +21,8 @@
  ***************************************************************************/
 #include "test.h"
 
+#include <string.h>
+
 /*
  * This test uses these funny custom memory callbacks for the only purpose
  * of verifying that curl_global_init_mem() functionality is present in