Fix std::string not declared error.
Fix std::string not declared error.

GPUOpen-LibrariesAndSDKs/RadeonRays_SDK,FlorentGuinier/RadeonRays_SDK,beasterio/FireRays_SDK,FlorentGuinier/RadeonRays_SDK,beasterio/FireRays_SDK,FlorentGuinier/RadeonRays_SDK,GPUOpen-LibrariesAndSDKs/RadeonRays_SDK,beasterio/FireRays_SDK,GPUOpen-LibrariesAndSDKs/RadeonRays_SDK