ggml-virtgpu : include missing mutex header (ggml-org#22810)

Add missing `#include <mutex>` in ggml-backend-device.cpp.

Fixes: ggml-org#22809

Signed-off-by: Oliver Walsh <owalsh@redhat.com>

Author: olliewalsh

ggml/src/ggml-virtgpu/ggml-backend-device.cpp

@@ -1,5 +1,7 @@
 #include "ggml-remoting.h"
 
+#include <mutex>
+
 static const char * ggml_backend_remoting_device_get_name(ggml_backend_dev_t dev) {
     virtgpu * gpu = DEV_TO_GPU(dev);