src/thread/pthread/SDL_systhread.c: drop include of SDL_platform.h

Drop include of SDL_platform.h as SDL_plaform.h is already included by
SDL_internal.h -> SDL_config.h -> SDL_platform.h

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>

src/thread/pthread/SDL_systhread.c

@@ -48,7 +48,6 @@
 #endif
 #endif
 
-#include "SDL_platform.h"
 #include "SDL_thread.h"
 #include "../SDL_thread_c.h"
 #include "../SDL_systhread.h"