[SYCL][NFC] Reintroduce missing Windows symbols (#6830)

#6685 unintentionally caused the
windows library to lose symbols for handler::verifyKernelInvoc and
handler::is_host. This patch introduces these as unused members to avoid
the patch being ABI-breaking.

Signed-off-by: Larsen, Steffen <steffen.larsen@intel.com>

sycl/include/sycl/handler.hpp

@@ -468,6 +468,9 @@ class __SYCL_EXPORT handler {
 
   ~handler() = default;
 
+  // TODO: Private and unusued. Remove when ABI break is allowed.
+  bool is_host() { return MIsHost; }
+
 #ifdef __SYCL_DEVICE_ONLY__
   // In device compilation accessor isn't inherited from AccessorBaseHost, so
   // can't detect by it. Since we don't expect it to be ever called in device
@@ -541,6 +544,12 @@ class __SYCL_EXPORT handler {
                        sizeof(sampler), ArgIndex);
   }
 
+  // TODO: Unusued. Remove when ABI break is allowed.
+  void verifyKernelInvoc(const kernel &Kernel) {
+    std::ignore = Kernel;
+    return;
+  }
+
   /* The kernel passed to StoreLambda can take an id, an item or an nd_item as
    * its argument. Since esimd plugin directly invokes the kernel (doesn’t use
    * piKernelSetArg), the kernel argument type must be known to the plugin.