[SYCL][DOC] Fix sample CMakeLists.txt to specify language (#7383)

This is necessary when using it on Windows and doesn't hurt on Linux.

See: #6026

Author: npmiller

sycl/doc/GetStartedGuide.md

@@ -735,7 +735,7 @@ cmake_minimum_required(VERSION 3.14)
 set(CMAKE_CXX_COMPILER "clang++")
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fsycl")
 
-project(simple-sycl-app)
+project(simple-sycl-app LANGUAGES CXX)
 
 add_executable(simple-sycl-app simple-sycl-app.cpp)
 ```