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/SDL_assert_c.h

@@ -19,6 +19,11 @@
   3. This notice may not be removed or altered from any source distribution.
 */
 
+#ifndef SDL_assert_c_h_
+#define SDL_assert_c_h_
+
 extern void SDL_AssertionsQuit(void);
 
+#endif /* SDL_assert_c_h_ */
+
 /* vi: set ts=4 sw=4 expandtab: */