msgpack: add dependency on atomic operations

This fixes:
http://autobuild.buildroot.net/results/44849a386cefb8899f4560296bfbbd8ea19ee910/



Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>
[yann.morin.1998@free.fr: use the new symbol; remove comment strings]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>