[SYCL] Fix unused plugin member after #15262 (#15312)

#15262 failed post-commit testing, this should fix it

Author: callumfare

sycl/source/detail/plugin.hpp

@@ -223,7 +223,9 @@ class plugin {
   // represents the unique ids of the last device of each platform
   // index of this vector corresponds to the index in UrPlatforms vector.
   std::vector<int> LastDeviceIds;
+#ifdef _WIN32
   void *UrLoaderHandle = nullptr;
+#endif
   UrFuncPtrMapT UrFuncPtrs;
 }; // class plugin