Consistently use angle brackets for SDL.h includes

That's how it should be done, because the SDL headers aren't going to be
installed in this repository. The game was a bit inconsistent before but
now it isn't anymore.

desktop_version/src/Textbox.h

@@ -1,7 +1,7 @@
 #ifndef TEXTBOX_H
 #define TEXTBOX_H
 
-#include "SDL.h"
+#include <SDL.h>
 #include <string>
 #include <vector>