Fix build with Qt dev (5.2)

QVariant now has an operator< that is ambiguous with this one.
(since commit 7ed15da3c1b9e2bb26f414a10bfc6e6d79d7cc7b in qtbase)

Change-Id: Idf38afe85cec966df1885aa2da0f37fbb3eeccf6
Reviewed-by: Marco Bubke <marco.bubke@digia.com>