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/KeyPoll.h

@@ -5,7 +5,7 @@
 #include <vector>
 #include <map> // FIXME: I should feel very bad for using C++ -flibit
 
-#include "SDL.h"
+#include <SDL.h>
 
 #include "Screen.h"