Update includes to to be relative to src directory

Don't use relative imports like `../foo.h` as those depend on the
relative position of both files. Rather than that use imports relative
to the `src` directory, which explicitly is part of the include
directories.

src/displayapp/screens/ScreenList.h

@@ -3,8 +3,8 @@
 #include <array>
 #include <functional>
 #include <memory>
-#include "Screen.h"
-#include "../DisplayApp.h"
+#include "displayapp/screens/Screen.h"
+#include "displayapp/DisplayApp.h"
 
 namespace Pinetime {
   namespace Applications {