zeromq: fix pkg-config file for static linking

zeromq: fix pkg-config file for static linking

Fixes:
http://autobuild.buildroot.net/results/faf/faf0bb00fccb1350afaef55f6cf14c11a5f5dc3c/
http://autobuild.buildroot.net/results/8d2/8d279baa4c8bffbb01f897119092c7b736942df4/
http://autobuild.buildroot.net/results/4d3/4d3dea604da9a5a1e7fe20548813f8de474ae33f/
http://autobuild.buildroot.net/results/fd6/fd602617839817352763e51754553960b7f795ac/

And many more.

Libzmq uses C++ standard library features, so users of it should link
against that as well when statically linking.

Add it to Libs.private so users using pkg-config automatically gets the
correct linker flags.

Patch is upstream:
https://github.com/zeromq/libzmq/pull/1398

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>