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/LittleVgl.cpp

@@ -1,5 +1,5 @@
-#include "LittleVgl.h"
-#include "lv_pinetime_theme.h"
+#include "displayapp/LittleVgl.h"
+#include "displayapp/lv_pinetime_theme.h"
 
 #include <FreeRTOS.h>
 #include <task.h>