Fixed warning C4152: nonstandard extension, function/data pointer conversion in expression

src/thread/SDL_thread_c.h

@@ -69,7 +69,7 @@ struct SDL_Thread
     int(SDLCALL *userfunc)(void *);
     void *userdata;
     void *data;
-    void *endfunc; /* only used on some platforms. */
+    SDL_FunctionPointer endfunc; /* only used on some platforms. */
 };
 
 /* This is the function called to run a thread */

src/thread/windows/SDL_systhread.c

@@ -82,7 +82,7 @@ int SDL_SYS_CreateThread(SDL_Thread *thread)
     const DWORD flags = thread->stacksize ? STACK_SIZE_PARAM_IS_A_RESERVATION : 0;
 
     /* Save the function which we will have to call to clear the RTL of calling app! */
-    thread->endfunc = pfnEndThread;
+    thread->endfunc = (SDL_FunctionPointer)pfnEndThread;
 
     /* thread->stacksize == 0 means "system default", same as win32 expects */
     if (pfnBeginThread) {