[SYCL][NFC] Align TryToGetNumElements and is_same_vector_size_impl definition and declaration to avoid compiler warnings

Compiling user code with -Wall provides compilation warnings from SYCL headers.
In some cases (-Werror or smth like that is enabled) it may not allow to compile user code.

```
CL/sycl/detail/generic_type_traits.hpp:468:1: warning: 'TryToGetNumElements' defined as a struct template here but previously declared as a class template; this is valid,  but may result in linker errors under the Microsoft C++ ABI [-Wmismatched-tags]
struct TryToGetNumElements<
```

Signed-off-by: Sergey Kanaev <[email protected]>

Author: Sergey Kanaev

sycl/include/CL/sycl/detail/generic_type_traits.hpp

@@ -462,7 +462,7 @@ using common_rel_ret_t =
 template <int N> struct Boolean;
 
 // Try to get vector element count or 1 otherwise
-template <typename T, typename Enable = void> class TryToGetNumElements;
+template <typename T, typename Enable = void> struct TryToGetNumElements;
 
 template <typename T>
 struct TryToGetNumElements<
@@ -561,7 +561,8 @@ template <typename T> static constexpr T quiet_NaN() {
 template <int FirstSize, typename... Args> struct is_same_vector_size_impl;
 
 template <int FirstSize, typename T, typename... Args>
-class is_same_vector_size_impl<FirstSize, T, Args...> {
+struct is_same_vector_size_impl<FirstSize, T, Args...> {
+private:
   using CurrentT = detail::remove_pointer_t<T>;
   static constexpr int Size = vector_size<CurrentT>::value;
   static constexpr bool IsSizeEqual = (Size == FirstSize);