Add and update include guards

Include guards in most changed files were missing, I added them keeping
the same style as other SDL files. In some cases I moved the include
guards around to be the first thing the header has to take advantage of
any possible improvements compiler may have for inclusion guards.

src/timer/SDL_timer_c.h

@@ -18,6 +18,10 @@
      misrepresented as being the original software.
   3. This notice may not be removed or altered from any source distribution.
 */
+
+#ifndef SDL_timer_c_h_
+#define SDL_timer_c_h_
+
 #include "../SDL_internal.h"
 
 /* Useful functions and variables from SDL_timer.c */
@@ -31,4 +35,6 @@ extern void SDL_TicksQuit(void);
 extern int SDL_TimerInit(void);
 extern void SDL_TimerQuit(void);
 
+#endif /* SDL_timer_c_h_ */
+
 /* vi: set ts=4 sw=4 expandtab: */