package/fio: bump version to 2.6, fix musl build

Fixes
http://autobuild.buildroot.net/results/063/0637236b89e3251bbf8b649ff60de0ff19494703/
http://autobuild.buildroot.net/results/2fe/2fefe784fb45368344775c7b6f2eb83e1c0c2104/


and others

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>