Fixed Windows RT build

src/thread/windows/SDL_systhread.c

@@ -29,6 +29,7 @@
 #include "../SDL_thread_c.h"
 #include "../SDL_systhread.h"
 #include "SDL_systhread_c.h"
+#include "../../core/windows/SDL_windows.h"
 
 #ifndef SDL_PASSED_BEGINTHREAD_ENDTHREAD
 /* We'll use the C library from this DLL */

src/thread/windows/SDL_systls.c

@@ -28,6 +28,13 @@
 #include "SDL_thread.h"
 #include "../SDL_thread_c.h"
 
+#if WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_APP)
+#define TLS_OUT_OF_INDEXES  FLS_OUT_OF_INDEXES
+#define TlsAlloc()  FlsAlloc(NULL)
+#define TlsSetValue FlsSetValue
+#define TlsGetValue FlsGetValue
+#endif
+
 static DWORD thread_local_storage = TLS_OUT_OF_INDEXES;
 static SDL_bool generic_local_storage = SDL_FALSE;