package/qt5/qt5base: fix musl build

Fixes
http://autobuild.buildroot.net/results/a15/a1586717154efb6495dfcdb5c7ce11a047dd68db/
http://autobuild.buildroot.net/results/363/3636ddcc0efa974c674f36d3c65eea5beb32af52/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>