Use C++ cast instead of old style cast

[texierp]

This to avoid the following warning with the '-Wold-style-cast' flag enabled :

./zmq.hpp:763:29: warning: use of old-style cast [-Wold-style-cast]
return (size_t) nbytes;
^
./zmq.hpp: In member function ‘size_t zmq::socket_t::recv(void*, size_t, int)’:
./zmq.hpp:793:29: warning: use of old-style cast [-Wold-style-cast]
return (size_t) nbytes;

[coveralls]

Pull Request Test Coverage Report for Build 209

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 48.751%

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Totals
Change from base Build 207:	0.0%
Covered Lines:	2089
Relevant Lines:	4285

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💛 - Coveralls

[sigiesec]

Thanks for the PR :)