Fix change-of-sign warning found by ICC

-1 is not a valid unsigned value.

registerhandler.cpp(354): warning #68: integer conversion resulted in a change of sign
          return createIndex(row, col, TopLevelId);
                                       ^

Change-Id: I41935255704f19724d6cec16ea470e2f8f8a16a1
Reviewed-by: Kurt Pattyn <pattyn.kurt@gmail.com>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>
Reviewed-by: hjk <hjk121@nokiamail.com>

src/plugins/debugger/registerhandler.cpp

@@ -293,7 +293,7 @@ static int bitWidthFromType(int type, int subType)
     return 0;
 }
 
-static const int TopLevelId = -1;
+static const uint TopLevelId = UINT_MAX;
 static bool isTopLevelItem(const QModelIndex &index)
 {
     return quintptr(index.internalId()) == quintptr(TopLevelId);