Add kernel32 to SDL_EXTRA_LIBS required for cross-compilation to Win32 native Linking error experienced while compiling with the following toolchain due to lack of kernel32.lib https://github.com/llvm-mirror/llvm/blob/d35e5f8dded4c884ca25ca88f184e1505cad106c/cmake/platforms/WinMsvc.cmake#L317-L321