Make a subtle include order change for SDL2.

Instead of compiling Wesnoth cleanly with a lot of linker errors I run
again in a new set of compilation errors. (Well I still consider this
progress ;-))

src/font.hpp

@@ -15,10 +15,11 @@
 #define FONT_HPP_INCLUDED
 
 #include "exceptions.hpp"
-#include "SDL_ttf.h"
 
 #include "sdl_utils.hpp"
 
+#include <SDL_ttf.h>
+
 class t_string;
 
 namespace font {