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/Motion.cpp

@@ -1,6 +1,6 @@
 #include <libs/lvgl/lvgl.h>
-#include "Motion.h"
-#include "../DisplayApp.h"
+#include "displayapp/screens/Motion.h"
+#include "displayapp/DisplayApp.h"
 
 using namespace Pinetime::Applications::Screens;