metal: initialize encode_async in ggml_backend_metal_init (#2334)

ggml_metal_init leaves ctx->encode_async nil, but
ggml_backend_metal_graph_compute invokes it unconditionally, so a Metal
backend created without a prior set_n_cb call segfaults on its first
graph. This affects rpc-server and ggml_backend_reg_metal_init; the
llama.cpp path is unaffected, since llama_graph_compute sets n_cb
before every compute.

Upstream added the same call in cad341d88 (#9698), the commit that
introduced encode_async. ik_llama.cpp carries the block but not the
initialization.

ggml/src/ggml-metal.m

@@ -4509,6 +4509,8 @@ ggml_backend_t ggml_backend_metal_init(void) {
         /* .context   = */ ctx,
     };
 
+    ggml_backend_metal_set_n_cb(metal_backend, 1);
+
     return metal_backend;
 }