Include SDL header as <SDL.h> instead of <SDL/SDL.h>.

SDL headers are located using "sdl-config --cflags", which outputs
the actual directory that the headers are in, not its parent.

src/inputmanager.h

@@ -21,7 +21,7 @@
 #ifndef INPUTMANAGER_H
 #define INPUTMANAGER_H
 
-#include <SDL/SDL.h>
+#include <SDL.h>
 #include <sstream>
 
 using namespace std;