ggml-backend : skip register metal backend on os simulator

Author: jhen0409

ggml/src/ggml-backend.cpp

@@ -575,8 +575,11 @@ struct ggml_backend_registry {
         register_backend(ggml_backend_cuda_reg());
 #endif
 #ifdef GGML_USE_METAL
+#include <TargetConditionals.h>
+#if !TARGET_OS_SIMULATOR
         register_backend(ggml_backend_metal_reg());
 #endif
+#endif
 #ifdef GGML_USE_SYCL
         register_backend(ggml_backend_sycl_reg());
 #endif