Fix deprecated literal operator

[cielavenir]

On clang 20, this happens:

$ clang++-20 zmq.hpp
zmq.hpp:1367:35: warning: identifier '_zbuf' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
 1367 | constexpr const_buffer operator"" _zbuf(const char *str, size_t len) noexcept
      |                        ~~~~~~~~~~~^~~~~
      |                        operator""_zbuf
zmq.hpp:1371:35: warning: identifier '_zbuf' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
 1371 | constexpr const_buffer operator"" _zbuf(const wchar_t *str, size_t len) noexcept
      |                        ~~~~~~~~~~~^~~~~
      |                        operator""_zbuf
zmq.hpp:1375:35: warning: identifier '_zbuf' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
 1375 | constexpr const_buffer operator"" _zbuf(const char16_t *str, size_t len) noexcept
      |                        ~~~~~~~~~~~^~~~~
      |                        operator""_zbuf
zmq.hpp:1379:35: warning: identifier '_zbuf' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
 1379 | constexpr const_buffer operator"" _zbuf(const char32_t *str, size_t len) noexcept
      |                        ~~~~~~~~~~~^~~~~
      |                        operator""_zbuf
4 warnings generated.