Added missing guards in implementation for PSP.

Thanks to Martin Gerhardy for pointing this out.

src/timer/psp/SDL_systimer.c

@@ -20,6 +20,8 @@
 */
 #include "../../SDL_internal.h"
 
+#ifdef SDL_TIMERS_PSP
+
 #include "SDL_thread.h"
 #include "SDL_timer.h"
 #include "SDL_error.h"
@@ -83,5 +85,7 @@ void SDL_Delay(Uint32 ms)
     sceKernelDelayThreadCB(ms * 1000);
 }
 
+#endif /* SDL_TIMERS_PSP */
+
 /* vim: ts=4 sw=4
  */