JANITORIAL: Reduce header dependencies in shared code

Some backends may break as I only compiled SDL

backends/timer/default/default-timer.h

@@ -28,8 +28,6 @@
 #include "common/timer.h"
 #include "common/mutex.h"
 
-class OSystem;
-
 struct TimerSlot;
 
 class DefaultTimerManager : public Common::TimerManager {

backends/timer/sdl/sdl-timer.cpp

@@ -30,6 +30,8 @@
 
 #include "backends/timer/sdl/sdl-timer.h"
 
+#include "common/textconsole.h"
+
 static Uint32 timer_handler(Uint32 interval, void *param) {
 	((DefaultTimerManager *)param)->handler();
 	return interval;