Didn't need to add SDL_windows.h include, that was already included

src/thread/windows/SDL_systhread.c

@@ -29,7 +29,6 @@
 #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 */