examples: fix missing header

[stephanlachnit]

With GCC12, I get this error:

../subprojects/cppzmq-4.8.1/examples/pubsub_multithread_inproc.cpp: In function ‘void PublisherThread(zmq::context_t*)’:
../subprojects/cppzmq-4.8.1/examples/pubsub_multithread_inproc.cpp:14:23: error: ‘sleep_for’ is not a member of ‘std::this_thread’
   14 |     std::this_thread::sleep_for(std::chrono::milliseconds(20));

This is because sleep_for is part of thread, which is not included in the example. This PR fixes that.

[coveralls]

Pull Request Test Coverage Report for Build 2846473152

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 86.037%

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Totals
Change from base Build 2504979042:	0.0%
Covered Lines:	801
Relevant Lines:	931

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💛 - Coveralls