python-pyzmq: ensure correct path to zmq is used

python-pyzmq: ensure correct path to zmq is used

Fixes:
http://autobuild.buildroot.net/results/6ae/6ae84c53985ab52ab1c64dd1e2a16d0bcf9eed1e/
http://autobuild.buildroot.net/results/04d/04d5ceb5335912916f05596a285e84d38a7c7d31/
http://autobuild.buildroot.net/results/77a/77ae85f24fabd00f2981f2dbddb52d8d5091b4a8/
http://autobuild.buildroot.net/results/27a/27adf12cca4e0e998b64fda9bf4be67f6dbe1595/

And many more.

If --zmq isn't passed then python-pymq falls back to using 'pkg-config
--variable=prefix libzmq' which returns /usr, causing it to look in /usr/lib
for libraries breaking the build.

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