[promise-endpoint] portability fix (#35435)

Fix error:

```
In file included from /[var/local/git/grpc/src/core/lib/transport/promise_endpoint.cc:17](https://cs.corp.google.com/piper///depot/google3/var/local/git/grpc/src/core/lib/transport/promise_endpoint.cc?l=17):
/[var/local/git/grpc/src/core/lib/transport/promise_endpoint.h:160](https://cs.corp.google.com/piper///depot/google3/var/local/git/grpc/src/core/lib/transport/promise_endpoint.h?l=160):22: error: call to deleted constructor of 'grpc_core::SliceBuffer'
              return ret;
                     ^~~
/[var/local/git/grpc/src/core/lib/slice/slice_buffer.h:53](https://cs.corp.google.com/piper///depot/google3/var/local/git/grpc/src/core/lib/slice/slice_buffer.h?l=53):3: note: 'SliceBuffer' has been explicitly marked deleted here
  SliceBuffer(const SliceBuffer& other) = delete;
  ^
/[var/local/git/grpc/src/core/lib/promise/poll.h:86](https://cs.corp.google.com/piper///depot/google3/var/local/git/grpc/src/core/lib/promise/poll.h?l=86):10: note: passing argument to parameter 'value' here
  Poll(U value) : ready_(true) {
         ^
```

seen here:
https://fusion2.corp.google.com/ci/kokoro/prod:grpc%2Fcore%2Fmaster%2Flinux%2Fgrpc_portability/activity/664ec53e-5509-4172-a6d1-182a3b4f5ad1/log

Closes #35435

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35435 from ctiller:pbe2 14de33646b
PiperOrigin-RevId: 595579651

src/core/lib/transport/promise_endpoint.h

@@ -157,7 +157,7 @@ class PromiseEndpoint {
               grpc_slice_buffer_move_first(read_state->buffer.c_slice_buffer(),
                                            num_bytes, ret.c_slice_buffer());
               read_state->complete.store(false, std::memory_order_relaxed);
-              return ret;
+              return std::move(ret);
             }
             read_state->complete.store(false, std::memory_order_relaxed);
             return std::move(read_state->result);