libatomic_ops: fix build failure seen with toolchain without threads support

Fixes:
http://autobuild.buildroot.net/results/58c188ad59c62c7897381e110fe3518ccc17867e



Signed-off-by: Gilles Talis <gilles.talis@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>