[GPU] Fix RapidJSON assertion failure during loading kernel (#7526)

Signed-off-by: Andrew Kwangwoong Park <andrew.kwangwoong.park@intel.com>

inference-engine/thirdparty/clDNN/kernel_selector/core/auto_tuner.cpp

@@ -317,6 +317,7 @@ void AutoTuner::RemoveKernel(const std::string& cacheFilePath,
 
 std::tuple<std::string, int> AutoTuner::LoadKernelOffline(TuningCache* deviceCache,
                                                           const Params& params) {
+    std::lock_guard<std::mutex> lock(mutex);
     static const uint32_t defaultComputeUnits = 24;
     if (!deviceCache)
         return {};