[Fix] Added missing #include (#34339)

To fix the following build error with the head of abseil

```
/var/local/git/grpc/test/core/tsi/ssl_transport_security_utils_test.cc:231:42: error: no member named 'StrCat' in namespace 'absl'
        return absl::InternalError(absl::StrCat("Client error:", client_err));
                                   ~~~~~~^
/var/local/git/grpc/test/core/tsi/ssl_transport_security_utils_test.cc:238:42: error: no member named 'StrCat' in namespace 'absl'
        return absl::InternalError(absl::StrCat("Server error:", server_err));
                                   ~~~~~~^
```

test/core/tsi/ssl_transport_security_utils_test.cc

@@ -29,6 +29,7 @@
 #include "absl/status/status.h"
 #include "absl/status/statusor.h"
 #include "absl/strings/match.h"
+#include "absl/strings/str_cat.h"
 #include "absl/strings/string_view.h"
 
 #include "src/core/tsi/transport_security.h"