[SYCL] Fix -Werror=unused-but-set-variable warning with old gcc (intel#18602)

Follow-up for intel#18477.

Author: aelovikov-intel

sycl/source/detail/device_impl.hpp

@@ -514,7 +514,7 @@ class device_impl : public std::enable_shared_from_this<device_impl> {
     // descriptor first when defining the cache data member). For "CallOnce"
     // cache we want to be querying cached value so "false" is the right
     // template parameter for such delegation.
-    constexpr bool DependentFalse = InitializingCache && false;
+    [[maybe_unused]] constexpr bool DependentFalse = InitializingCache && false;
 
     if constexpr (decltype(MCache)::has<Param>() && !InitializingCache) {
       return MCache.get<Param>();