wasi_ephemeral_nn.h: add a convenience wrapper header

Author: yamt

core/iwasm/libraries/wasi-nn/include/wasi_ephemeral_nn.h

@@ -0,0 +1,7 @@
+/*
+ * Copyright (C) 2025 Midokura Japan KK.  All rights reserved.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ */
+
+#define WASM_ENABLE_WASI_EPHEMERAL_NN 1
+#include "wasi_nn.h"