moc: replace offsetof with sizeof

It's the same thing since we have exactly one field in this structure
before stringdata0. But there's a far smaller chance of producing a
warning in user code.

Pick-to: 6.2 6.3 6.4
Change-Id: Id0fb9ab0089845ee8843fffd16f9cdceec33017d
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/tools/moc/generator.cpp

@@ -254,7 +254,7 @@ void Generator::generateCode()
     // Macro that simplifies the string data listing. The offset is calculated
     // from the top of the stringdata object (i.e., past the uints).
     fprintf(out, "#define QT_MOC_LITERAL(ofs, len) \\\n"
-            "    uint(offsetof(qt_meta_stringdata_%s_t, stringdata0) + ofs), len \n",
+            "    uint(sizeof(qt_meta_stringdata_%s_t::offsetsAndSizes) + ofs), len \n",
             qualifiedClassNameIdentifier.constData());
 
     fprintf(out, "static const qt_meta_stringdata_%s_t qt_meta_stringdata_%s = {\n",