[SYCL][NFC] Align type_list definition and declaration to avoid compiler warnings (#983)

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/type_list.hpp:34:13: warning: 'type_list' 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]
template <> struct type_list<> {};
```
Signed-off-by: Sergey Kanaev <sergey.kanaev@intel.com>

Author: s-kanaev

sycl/include/CL/sycl/detail/type_list.hpp

@@ -22,7 +22,7 @@ template <typename T> using head_t = typename T::head;
 template <typename T> using tail_t = typename T::tail;
 
 // type_list
-template <typename... T> class type_list;
+template <typename... T> struct type_list;
 
 using empty_type_list = type_list<>;