QT: Fix build for architectures using generic atomics

The problem has to do with const-correctness. This has been resolved
for various architectures, but not for the generic case.

Signed-off-by: Mischa Jonker <mischa.jonker@synopsys.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>