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/Steps.cpp

@@ -1,7 +1,7 @@
-#include "Steps.h"
+#include "displayapp/screens/Steps.h"
 #include <lvgl/lvgl.h>
-#include "../DisplayApp.h"
-#include "Symbols.h"
+#include "displayapp/DisplayApp.h"
+#include "displayapp/screens/Symbols.h"
 
 using namespace Pinetime::Applications::Screens;