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/NotificationIcon.cpp

@@ -1,5 +1,5 @@
-#include "NotificationIcon.h"
-#include "Symbols.h"
+#include "displayapp/screens/NotificationIcon.h"
+#include "displayapp/screens/Symbols.h"
 using namespace Pinetime::Applications::Screens;
 
 const char* NotificationIcon::GetIcon(bool newNotificationAvailable) {