set mmap_support to false on OpenCL backend

ggml/src/ggml-opencl/ggml-opencl.cpp

@@ -10777,7 +10777,7 @@ static void ggml_backend_opencl_device_get_props(ggml_backend_dev_t dev, struct
         /* .host_buffer           = */ false,
         /* .buffer_from_host_ptr  = */ false,
         /* .events                = */ false,
-        /* .mmap_support          = */ true,
+        /* .mmap_support          = */ false,
     };
 }