Fix includes for Boring SSL.

src/core/tsi/ssl_transport_security.c

@@ -34,6 +34,7 @@
 #include "src/core/tsi/ssl_transport_security.h"
 
 #include <limits.h>
+#include <string.h>
 
 #include <grpc/support/log.h>
 #include <grpc/support/sync.h>