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

CMakeLists.txt

@@ -2018,7 +2018,7 @@ elseif(WINDOWS)
 
   # Libraries for Win32 native and MinGW
   if(NOT WINDOWS_STORE)
-    list(APPEND SDL_EXTRA_LIBS user32 gdi32 winmm imm32 ole32 oleaut32 version uuid advapi32 setupapi shell32)
+    list(APPEND SDL_EXTRA_LIBS kernel32 user32 gdi32 winmm imm32 ole32 oleaut32 version uuid advapi32 setupapi shell32)
   endif()
 
   if(WINDOWS_STORE)