czmq: fix static build

libstdc++ may also need -lm.

Fixes:
http://autobuild.buildroot.net/results/aab/aab80285c54641fd7c217cdd45c4593e5c3a66c5/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>