Restore String size assert

Just like all of the other datastructures in qv4compileddata, changing
the size of String would require an inrcement of
QV4_DATA_STRUCTURE_VERSION.

Change-Id: I462d04204255946d309d245bc20b044ba9d222d7
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

src/qml/common/qv4compileddata_p.h

@@ -227,6 +227,8 @@ struct String
     }
 };
 
+static_assert (sizeof (String) == 4, "String structure needs to have the expected size to be binary compatible on disk when generated by host compiler and loaded by target");
+
 struct CodeOffsetToLine {
     quint32_le codeOffset;
     quint32_le line;